You can attach files by going to the efile menu at the top of your screen and selecting "attach pdf file". Note that not all "other" pdfs are acceptable especially when it comes to certain states, and alot of forms have a "supporting statement" line where applicable.
There is also a doc on the Federal 1040 called "Miscellaneous Statement" where you can document entries as well.
If you attach a pdf to return it probably will not be looked at by the IRS. Better to put it on preparer notes, which is efiled with the return, You can find it by pressing F6, navigate to Miscellaneous, and click on it, then press OK. Go to part 2, line 3 and quickzoom to preparer notes