Many people who print PDFs simply want to be able to make notes on the pages, and don't realize that this can readily be done digitally. If you print the PDF and then mark it up, you then have to deal with a bunch of sheets of paper that you have to keep in order, which can get mangled, and whose markups cannot later be altered.
PDFs are readily annotated. Many examples of doing this can be found on YouTube by searching for "annotate pdf".
The Macintosh offers one of the best PDF annotation applications available, and it is included free with OS X: it is the Preview application. PDF is the standard document model in OS X, and with Preview you can do all kinds of things with PDFs, including annotation. You can highlight text (like yellow marker), underline text, strike through text, draw an oval or rectangle around page contents, add bubble text, add notes. You can accomplish annotation using menu options, but the easier approach is to add elements to the toolbar: right click on the toolbar, choose Customize Toolbar and then drag the Markup and Edit tools into it, and click Done. To then reveal your annotation tools, click on the Edit icon (Show Edit Toolbar). Now, by clicking on one of those tools on the left side, such as Oval, you make it active for your editing, until you click that took again to deactivate it.
PDF annotation is readily accomplished on the iPad as well, where there are numerous apps available for the purpose, including iAnnotate and PDF Expert.
Get into the PDF annotation habit, and stop wasteful printing.