Lots of materials on the web are in pdf files. This is a format created by Adobe Systems primarily to guarantee how a document would print out. You may have noticed that when you look at a regular web page, it can appear quite different on different machines, depending on which fonts are installed, the display set-up, and so on. Printing out can be even more of a challenge. The pdf format solves this.
Eventually, it became a de facto standard. See more information in Wikipedia on Portable Document Format.
PDFs have become very popular for scanning, for example, Google Books uses it almost exclusively. Many people use it just to print however, and this can result in a huge waste of paper. One of the problems of pdfs is that you cannot add notes or mark them up like you can a piece of paper. Or, you can, but you must pay Adobe for the program that allows you to do it.
PDF Xchange Viewer changes all of that. It is a free viewer that is much faster than the viewer put out by Adobe, and it allows you to add your own notes and mark up your text in various ways. Best of all, it is free.
It is available from several sites on the web, but here is the one at CNet, always a good place to trust to download software. Be aware that the creator of a pdf file can disallow people the permission to mark up the text, but most of the time, there is no problem.
Give it a try. Here's an example file that I took from Google Books of a famous book of Edgar Allen Poe's and marked up the title page. While you can view all of the changes in the Adobe Acrobat Viewer, you can't make them or change them.
You may find PDF Xchange Viewer useful and, you might even save a tree or two.