Calibre is a  program that runs in many different PC environments to manage a database of eBooks. From there, you can convert books between formats, and upload books into cell phone apps, iPads,  and standalone eBook readers like Amazon's "Kindle" and the Barnes & Noble "Nook". On the Android phone, the eBook reader app is called: Aldiko)

eBooks come in multiple formats, and some are copy protected.  Copy protected books must be read in the corresponding reader program. If you buy an eBook from iTunes, Amazon, or Barnes & Noble and it is protected, you will need to use their reader software.

If you can, add unrestricted books to your library in ePUB format. An ePub contains several components (like a ZIP file) and can support books with full color pictures, fonts, table layouts, etc - just like HTML web pages. ePUB is positioned to be the future standard for eBooks.

If you have no choice, PDF documents are ok, and most readers will display them. Don't try to convert them to ePUB unless you absolutely have to. PDFs convert poorly as they are documents that are already prepped to be printed. They are "page image" files.

Other eBook formats (the old Microsoft LIT format and the old Palm PDA format called "PDB") will probably convert with little trouble to the ePUB format, but content in these older formats will not have pretty fonts and graphics will be limited - since eBooks "way back then" did not support hi res pictures.

Calibre will support, read and manage all of these eBook formats, and it is a free download.

http://calibre-ebook.com/download

To Get started with eBooks, download this classic, "The Nature and Causes of The Wealth of Nations Books I-V" - Adam Smith