Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
I've tried a bunch of the apps mentioned, but one of the simplest and most useful ones I found for my purposes is an app called "Files: Document Reader". I can send any file (pdf, movies, whatever) through the browser on my computer wirelessly to my iPhone, or it can access a number of servers, like dropbox, box, etc. Its also got its own password protection, built in browser (separate from safari), can import/export photos to your camera roll, and lots more. Its free, but you have to pay (IAP) to get some advanced features.
 
Last edited:
you can enable "Sync with this iPad over Wi-Fi" when your iPad is connected with iTunes.
Screen Shot 2016-01-08 at 5.36.11 PM.png

now your iPad will appear in iTunes when it is on the same Wi-Fi network.
you can sync (transfer) any media file (including pdf) wirelessly.
no apps required.
 
I've been using iCloud Drive with PDF Expert for this. I keep all my PDFs arranged in the Readdle folder and requires no physical sync.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.