Optimizing your PDF files can reduce your filesizes to a tenth or less of the original. This will make your pages download ten or more times as fast.
Use text instead of images
If you have scanned in a text document, use OCR (optical character recognition) to convert the scan from an image to text. Most scanners will come with some form of OCR software. If you have created the text in a separate application, do not convert the text to an image. Leave it as text.
If you have Adobe Acrobat Pro, it can convert your scanned document to text. Look under the Document menu for either “Recognize Text OCR” or “Paper Capture”. You’ll most likely want to capture to the PDF Output Style “Formatted Text and Graphics”.
Text is much easier to read and it is much easier to compress than are images that look like text. Readers who have trouble seeing can resize text, but they cannot resize an image--it just pixellates. Non-graphical readers such as voice readers can read text, but not images that look like text.
You can also decrease the size of a PDF document by using the standard fonts. The standard fonts are Courier, Helvetica, and Times. However, even non-standard fonts that must be embedded will generally result in smaller PDF files than converting that text to an image.
Use vector graphics instead of bitmap graphics
Vector graphics are like a text form of images: rather than specifying each pixel in the image, they specify only circles, lines, arcs, and other shapes. This tends to not only be smaller than the same image as a “bitmap” (jpeg, png, or gif) version, but it also compresses better.
Adobe Illustrator is an application that creates vector images. Adobe Photoshop is an application that creates bitmap images. Other applications that create vector images include Inkscape, Macromedia Freehand, and the “draw” portions of OpenOffice and AppleWorks.
Clean up and optimize your file
The instructions here are for Adobe Acrobat 6 Professional. On Mac OS X, the Preview application can “Reduce File Size”. Look for the option under “Quartz Filter” when you “Save As”.
Under Acrobat’s “Advanced” menu, choose “PDF Optimizer”. This will bring up a dialog box with three tabs of options: images, fonts, and clean up. Unless you have previously modified this screen, the important tab is the clean up tab. Check every option that does not specifically apply to your document.
When you’ve selected all of the options, choose the “OK” button to save your file. Give it a new name (don’t erase your original), and then check the filesize. If the new filesize is smaller, use the new file on your web site.
As an example, the USD campus map went from a 2,600 kilobyte file created with Illustrator, to a 560 kilobyte file--that’s a five times faster download.
You should generlaly be using JPEG for color or grayscale images, and CCITT Group 4 for monochrome images. You can adjust the pixels per inch as desired, but 72 pixels per inch should be enough for web quality. You may allow some leeway if you expect your images to be blown up to a higher size on the client, but should use no more than 150 pixels per inch, and that can significantly degrade the viewing experience of the majority of your viewers who do not need to blow up your document.
These are the recommended settings for Acrobat 6 on the Macintosh: