What is PDF?
The Portable Document Format (PDF) was developed by Adobe in 1993 as a standard for document files. At the time, TIFF was the standard; it could be read by desktop publishing programs, fax programs, and more. However, TIFF files are images: a facsimile of the original, or "fax." The content transferred is not in the original medium, so it has limited value. Image files are much larger than text files, so Adobe decided TIFF was not a good format for documents and developed the PDF format a year after TIFF was accepted as the agreed-upon industry standard.
Why Use PDF?
End-users' browser settings override your preferences when you publish to HTML, TXT files lose their formatting, but PDF keeps every detail - including pictures and their placement, font, and layout. Further, PDF is platform independent - your document will look the same on a PC as it does on a Macintosh or Linux. With PDF, you can be sure your content reaches your audience exactly the way you had intended - not compromised by end-user settings, platform issues, or other technical concerns. PDF is now open-source, too!
Why Compress PDF?
One of the major concerns already touched upon is that of file format support. Not all files can be read or written to under all operating systems. Archiving your file compresses it to a more manageable size, but the archiving format you use might not be supported by all operating systems. Though PDF files are almost always smaller than images, PDF file compress options can make them even smaller. Smaller files can be delivered faster, searched quicker, and so on. A good PDF file compress - a program that compresses PDF files further without a major loss in quality - makes PDF files easier to work with, faster to deliver, and more efficient all around.