These logos were created by Martin Holmes in 2013, based on 
these images in the public domain from Wikimedia Commons:

https://commons.wikimedia.org/wiki/File:Amazon_Kindle_logo.svg
https://commons.wikimedia.org/wiki/File:Adobe_PDF_Icon.svg

This image from Wikimedia Commons available under the Creative 
Commons Attribution-Share Alike 3.0 Unported license:

https://commons.wikimedia.org/wiki/File:Amazon-icon.png

This image from Wikimedia Commons which is non-free, but can be
used for purposes of identification:

http://en.wikipedia.org/wiki/File:EPUB_logo.svg

This image by Ronoaldo from OpenClipart.org:
http://openclipart.org/people/ronoaldo/ronoaldo_New_Document.svg

The images created by Martin Holmes are in the public domain 
except insofar as restrictions on the source images might be 
deemed to propagate to the derivative works.