Hi to open books with FBReader:
- Go to Menu | Books | Select the Book you want to read | Press and hold on the middle OK button
- A mini menu will pop up, select Open With
- Click to the right and select fbreader
Once you have done this once, your choosen ebook will open using FBreader every time unless you tell PocketBook otherwise.
I don't really read .pdfs but I really like how .epub files are displayed using FBReader and have had no problems. Of course it depends on the type of books you may be reading and the formatting.
I would ask about the fb2 conversion in the Calibre section here:
https://www.mobileread.com/forums/forumdisplay.php?f=166
Hopefully someone will be able to help with that.