Hi, I am a noob to tablets but might be able to answer one of your questions. I just purchased a Galaxy Tab S 8.4" at Best Buy this morning. Early Christmas present for me, LOL. The lady who helped me said the Tab Pro (which I looked at also) and the Tab S are pretty much alike; the Tab S is just the Tab Pro with the new bells and whistles. I went with the Tab S because of the Super AMOLED display as I'm an artistic type who can't resist the lure of what the AMOLED is supposed to be capable of.
Okay, so. Regarding the USB and flash drive - there is an adapter you can buy for about $20 that will let you plug flash drives and the larger SD card adapter or card in. I'm going to get one soon - would've gotten it today but I didn't get the credit approval so I had to put some of the accessories back and just buy the tablet. I'll get the adapter soon. See here for info about the adapter: http://www.gsmarena.com/samsung_galaxy_tab_pro_8_4-review-1042p10.php
I don't know about PDF yet but if I find out anything I'll be sure and report back! Good luck with your new tablet and wish me luck with mine, okay?