There are tens of thousands of photographs, if not more, of military vehicles and related subjects to be found on-line, but web pages really are not well-suited for printing out the photographs on them. They have the distinct drawback that you can only easily view them when you're at your computer—and for most people, this will not be while building a model. You can print them out, of course, but this is generally a bit of a hassle for any number of reasons: printing them one at a time, because printing straight from a web browser is often less than ideal; despairing at the small size, and so at details disappearing in the pixels; poor quality, because the person putting up the pictures may not actually know anything about computer images; and so on.
These are some of the reasons for using PDF files instead for this site. They're set up like real, hardcopy books and so contain multiple, high-quality photographs together with short histories, backgrounds, photo captions, and clarifying illustrations. What's more, you can print out PDFs quite easily, and they'll look just as good as they do on your screen.
See for yourself: the image at right above is a page spread taken directly from the net.book on the IS-2M heavy tank—except, of course, that it's been reduced in size quite a bit :) Both pages are A4 size, or 29.7 by 21 cm, and the photographs' resolution has been chosen to create a good balance between file size (for easy downloading) and definition of detail when the document is printed out. At left is an example of the kind of computer artwork used to further illustrate the documents, from the net.book about German bunkers. These illustrations serve to make clear what part of the real thing each photograph actually shows—take a close look at the page spread above, and you'll see a little crosshair on the small illustration on the left-hand page to point out exactly where the items photographed on the opposite page are on the real thing!