Apple's Preview is more than just a fast and efficient program for reading PDFs, viewing graphics, and running slide shows. The version that comes with Mac OS X 10.5 (Leopard) has some powerful features stashed away in its menus. Now you can enhance your images, annotate and merge PDFs, print multiple images on one page, and more – all without opening a specialized (and often expensive) image editor or PDF tool. Read on to discover eight great things you can do with Preview.

1. Annotate PDFs
One of the best things about Adobe's Portable Document Format (PDF) is that it allows you to share documents while making sure that everyone sees the same layout, even if the recipients don't own the software that created the document. You can create PDF files from the Print dialog box of any Mac OS X program and then send them to friends and colleagues. In the past, if you had detailed comments about the PDFs your colleagues sent you, you were stuck – you couldn't change the text, so you had to put your suggestions in another file, such as an email message. Alternatively, someone in your group had to buy an expensive copy of Adobe's Acrobat Professional 8 to allow you all to add comments directly to the file.
But Leopard's Preview lets you add notes, highlight and strike through text, or use ovals and rectangles to call attention to specific sections of the page. You can even add links to other pages in a document or to Web sites. Best of all, other PDF readers – including both Mac OS X and Windows versions of Adobe Acrobat Reader – can view all of these annotations.

To begin, open any PDF in Preview and then choose Tools: Annotate. A submenu gives you four options: Add Oval, Add Rectangle, Add Note, and Add Link. Use either the oval or the rectangle shape to draw attention to specific elements on the page (see "Share Comments"). Select the one you want in the menu and then click and drag to draw. Select the shape on the page and drag it around to position it precisely, or drag one of its corners to resize it.

Share Comments: Preview lets you mark up your documents through a variety of annotation tools, including ovals and notes, as you see here.To add comments, choose Add Note and then click where you want the note to appear; you'll see a small yellow icon that looks like a speech bubble. Place this wherever you like. Enter your text in the yellow box that appears in the margin. To move a note's icon later, double-click on it. A dark box around the icon means that it's selected. You can then drag it to a new location.

Want to link to a Web page that explains something in more depth, or to a note or table at the end of a document? Choose Add Link and then click and drag to create a link box over the text or area you want to link to something else. (You can also select some text and then choose Add Link, to have the link applied to the selected text.) When you do this, an Inspector appears, with the Annotations tab selected. Click on the Action menu and choose either Link Within PDF if you want to link to another page in the document, or URL to link to an external Web page. To complete the link within your document, scroll to the page you want as the target and then click on Set Destination. To link to a URL, type the full address in the URL text field (including http:// ) and click on Set URL. Save and close the file, and when you reopen it, the link will be active.

One big downside to using Preview's links is that they aren't indicated in PDFs. (If you happen to pass a cursor over the spot, it will change from an arrow to a hand.) Make sure readers don't miss yours by underlining the text or highlighting the area with an oval or rectangle.

To mark up the text, select some text in the document and then choose Tools: Mark Up. (Hold down the option key while selecting, to choose text in a column.) Pick from options including Highlight Text, Strike Through Text, and Underline Text. Unfortunately, you can't replace or add text in-line (in other words, you can't replace or directly add to any part of the text). You have to indicate those changes in a note.

If you need to edit your annotations later, open the document again, choose View: Sidebar, and then, from the pop-up menu at the bottom of the sidebar, choose Annotations. Click on one of your annotations in the sidebar to make it active; you can now edit, move, or delete it. To edit links, select the Include Links option at the top of the sidebar, and click on a link to edit it in the Inspector.

2. Delete or Rearrange PDF Pages
When you open a multipage PDF in Preview, you may find that you want to delete extraneous pages or change the order of pages. You can do this easily: display the sidebar if it's not visible (press Command-shift-D if it's not), and then select a page and press Command-delete. To move a page, select it and drag it to the desired location. You can even add blank pages, if you want: select the page after which you want to add a blank page, and then choose Edit: Insert Blank Page.

3. Merge PDFs
What if you have two PDFs, and you'd like to make a single document or move pages from one file to another? Before Leopard, you could do this only in Acrobat Professional or with a utility like MonkeyBread Software's free Combine PDFs. Now Preview makes it a piece of cake. Open both PDFs and make sure the sidebar is visible. Drag a page from one document's sidebar to another, and drop it in the appropriate location. To add an entire document, you can drag the file's proxy icon (the icon in the title bar) to any location in the other document's sidebar.

4. Crop and Resize Images
Say you need to quickly resize or crop an image to send to a friend or post on your blog. There's no need to open an image editor – Preview's image-editing features can do the trick. To crop an image, open it and then click on the Select button in the toolbar (if you don't see a square on this button, click and hold it and choose Rectangular Selection). Click and drag to create a rectangle around the area you want to crop. You can resize this rectangle by dragging one of its corners, or you can move it if you need to. Then choose Tools: Crop or press Command-K. If you like what you see, save the file; if not, press Command-Z to go back to the original image.

You can then resize your image and even change its resolution. Choose Tools: Adjust Size to enter a width or height in pixels, inches, or other units (choose the unit you want from the pop-up menu to the right of these fields). To maintain the image's proportions, make sure to select the Scale Proportionally option. To make an image smaller without losing quality, keep Resample Image selected.

5. Quickly Mask Part of Your Image
Sometimes you have a picture of a person or an object that's in front of a confusing background, and you'd really like to see that person without the clutter. Preview has two nifty features that let you do this – Instant Alpha and Extract Shape. To use the first, open a picture and then click and hold the Select button in Preview's toolbar. From the menu that appears, choose Instant Alpha.

Next, click and drag to remove background areas of the image. Start by dragging small areas to see how Preview expands the selection. You don't have to get everything in one pass – just click and drag again to add to your selection. If you make a mistake, press the escape key to remove the selection altogether or option-drag to deselect particular areas. When you've highlighted everything you want to remove, press return to crop the background from your picture.

Using Extract Shape is even easier. Choose it from the Select button menu, and then start drawing a line around the item you want to retain in your picture. As you do this, a thick red line follows your cursor. Make sure to draw entirely around the item you want to keep; continue until you've joined the beginning and end of your line. You'll then see a number of handles on the line.

Move any of these if you need to improve part of your outline. Don't worry about drawing perfectly. The magic starts when you press return – Preview calculates what to keep, and removes your picture's background. In many cases, you'll be able to crop out all the dreck the first time, though with some pictures you may need to repeat the process a couple of times.

6. Adjust Image Colour and More
Adjust Colours: Don't bother opening an image editor when you need to make a few enhancements to your picture. Preview's Adjust Color palette lets you tweak colors, brightness, and more. Programs such as Apple's iPhoto offer tools to adjust image colour, brightness, and exposure, but what if you simply want to adjust a single image without adding it to your iPhoto library, or you don't have the iLife suite. Open the image in Preview and then choose Tools: Adjust Color. The tools in this palette let you tweak your picture's color balance and brightness and even add a sepia tone to it (see "Adjust Colors"). Move the different sliders to see, in real time, what your picture will look like if you save your changes. For pictures that are too dark or overexposed, try clicking on the Auto Levels button to see if that fixes them.

7. Print Multiple Images per Page
It's a shame to waste a lot of paper printing a handful of images one to a page. While many programs let you print a bunch of images on a page, they typically require multiple steps to do so. Preview has a printing trick that does it automatically. First, put all your images in a single Preview window. If you already have one image open, display Preview's sidebar and then drag the other images from the Finder to the sidebar. If you don't have any images open, you can open them at once by dragging them all onto Preview's icon in the Dock or in the Finder. Next, select all the images in the sidebar by clicking on one and then pressing Command-A. Now choose File: Print Selected Images, or press Command-P. In the dialog box that appears, choose the number of images you want on each page from the Images Per Page pop-up menu. (Don't see this? Reveal more options by clicking on the disclosure triangle to the right of your printer's name.) You can choose to have multiple copies of each image if you check the Print N Copies Per Page option; otherwise, each image is printed once.

8. Add Keywords to Your Images or PDFs
Spotlight keywords make it possible for you to find certain files easily by using a special keyword that other files won't contain. For example, if you add the keyword REPVOLC to that report you are writing about volcanoes, Spotlight will find it instantly.

You can add Spotlight comments to any file by selecting it in the Finder, pressing Command-I, and typing in the Spotlight Comments field. But if you send files by email or iChat, or even just transfer them over a network, comments added in this way disappear. To add keywords permanently in Preview, open the file and select Tools: Inspector (or press Command-I). In the palette that appears, click on the Spotlight tab. Click on the plus sign (+) to add a new keyword, and then type your text. You can add as many keywords as you want. (Note that this doesn't work for images in PNG format.) When you're done, save the file. Now these keywords will stay put, no matter where the file travels. You'll find them in the Finder's Info palette (Command-I) in the More Info section, listed under Keywords.