I use an app that allows me to create folders in a folder to mimic an hierarchy I use in my PC. I can store PDF's, txt, and other files. I have files it cannot open stored on it. I do that for extra backup and if I am at a firend's house and I want her to test a stitch out, I can email the file to her while there and she can out it on her PC.I'd rather use other apps, that sync between operating systems. That way you can access them everywhere. iBooks is Apple only, and afaik PDF files are not included in every backup method and could be lost easily. That's just my opinion. Everyone has to decide for himself what's best.