I haven't had the need to create PDFs, but I believe they have one distinct advantage over images. People can use their favorite word processor or other applications, then create a PDF of it. It looks exactly like the original and all the viewer needs is a PDF reader.
So far it is similar to an image, but with PDF you can modify the PDF afterwards much more easily than you would be able with an image editor. If the file would be an image, you can't really modify it without decent skills in some decent image editor such as Photoshop. Also, PDF is good as a fill-out form. You can simply type what you want in designated areas, then print the form or send it as email.
In a nuthsell, S-99 is right in that PNG could do what PDF does, but PDF does the job it was designed to do much better because whoever creates, modifies and fills out PDFs needs much less skills to do so in PDF than PNG. Using image editor to fill out forms in PNG format would be more difficult because you can't just point, click and start typing. Nor can you edit already existing PNG file as easily as you would a PDF if you would have to, say to change a sentence. You would always have to keep the source material at hand if you ever think you have to change the PNG, otherwise it'll be a pain in the ass.
Sorry S-99, but advantages of PDF far outweights the minor annoyances of Adobe Reader. That said, PDF format is standardized and under OS X and linux you can view it without much hassle. Who knows, maybe Windows 7 image viewer will support PDFs natively.
I don't know how Vista ended up in this discussion, but you can disable desktop composition without reverting back to the ugly Windows 2000 theme by unselecting desktop composition in performance options. Still. it shouldn't be necessary unless it is a typical office workstation with sucky integrated graphics. If a computer has dedicated video card with its own physical RAM, it probably needs newer drivers. Vista composition is spiffy on my home computer and I like it.