I use a Nook charger with no issues, nor modifications. Puts out 2 amps. Found it at one of those weird stores that sells cheap the confiscated/lost/left behind items from airlines/TSA.

OK, I know this may well be a PITA, but maybe it's worth trying.
What about renaming all the .pdf files as .txt (or something equally "innocuous" for example), then see if they will open on the N7. You may have to download the .txt/.pdf you want to the N7, rename it to .pdf, then view it.

There was a recent update to the Dropbox app, that specifically mentioned improvements in video streaming (on the n7, IIRC too).
I did the update, but haven't tried it out sufficiently to comment on whether it's done the job.
I did ascertain that even with the update it wouldn't play some videos that also wouldn't play before the update.........despite all the files being encoded the same way. That's a mystery to me.
Skydrive reportedly streams movies with no issues BTW, though I don't currently use that platform.

Closing Chrome Tabs: there's a small "X" in the right side of all tabs, click the "X" to close the tab(s). AFAIK, each tab has to be closed individually, there is no global close function.