Calibre (software)

Calibre is a cross-platform free and open-source suite of e-book software. Calibre supports organizing existing e-books into virtual libraries, displaying, editing, creating and converting e-books, as well as syncing e-books with a variety of e-readers.
— Wikipedia

I use Calibre to convert books into required formats, and manage my books.

I also use custom web app for accessing my books across different devices.

Calibre providing special ebook converting tool, you can for example convert EPUB files into PDF file:

# ebook-convert also have recipes (news, websites, etc.), which can be used to
# convert websites into ebooks.
ebook-convert input.epub output.pdf
ebook-convert --list-recipes
ebook-convert "NASA.recipe" "NASA.epub"