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.
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