Easiest way I use is installing a good eBook reader which simply accesses files you place in its directory (in my case, Moon+ Reader in its payed version, currently 50% off its price). Then I can easily transfer my reading material (Moon+ supports a bunch of formats, including PDF, ePub, Mobi, FB2 and many more) either...
- via CardReader
- via USB (when connected)
- wirelessly (e.g. using a sync app like FolderSync, or a network file manager like ES File Explorer, or a remote management tool like Airdroid)
- via Dropbox is also available according to the description (never tried, have no Dropbox account)
You even can highlight and annotate (with PDFs, this goes directly into the PDF file; for other formats it's stored separately), integrate dictionaries like ColorDict, and much more.