I wouldn't do that. Almost all the tax returns that are filed by mail are printed on only one side. That's what the people who enter the returns into the computer are used to. If you print on both sides, there's too much chance that they will accidentally skip a page. Also, pages from different forms shouldn't be back to back on one sheet, and if a form is only one page, it should be on a sheet by itself. So you can't just print the whole PDF double-sided.