For best results printing the PDFs, use Avery business card paper. The Avery products listed in the table below are particularly well suited for flashcards because they are designed to be printed on both sides. If you are printing the A4 PDFs, I can tell you that Avery product number X works well with these files. Thanks to Helena Williams in Australia for sending me some A4 size paper to test with.
If your printer doesn't automatically print double-sided, use Adobe Reader to print the even-numbered pages in reverse order first. Then, load the printed pages back into the printer's feed tray. Print again, this time the odd-numbered pages in forward order. Try this with 3 or 4 pages first, before printing an entire set of flashcards.
Some Avery products have a waxy surface on the back side; this does not work well for double-sided printing. The PDFs now have characters on the front and back of each flashcard, so if you are having a hard time printing double-sided, then just print the back sides (the even-numbered pages).
With the exception of PCR books 1 and 2, these lists are released to the public domain. If you use them for a project other than personal study, please give this site credit. PCR books 1 and 2 are released by Matti Tukiainen under the terms of the GPL.