I dont remember if I suggested Hyphen... its epub only but it syncs reliably between devices (iOS devices, not others). There's a free version, which is linked at the HYphen website
http://hyphenreader.com/ and this is what I use *all* the time, in iOS, even though I have spent many dollars trying others out.
Then, its Kindle app for mobi/azw etc, and something else for PDF. Its not a massive effort to switch between apps for whichever you read.