chess openings

Pdf is not an appropriate format for tree-like structures.

It appears to me that there is good reason to fear that there will never again be another opening encyclopedia. As far as I know, the last one was about seven years ago.
https://web.archive.org/web/20140627063241/http://www.chesscafe.com/text/hansen145.pdf
From time to time, there has been a book that attempts to introduce the openings without including tables of variations. One such book, Fundamental Chess Openings by GM Paul van der Sterren, appeared about eight years ago.
https://web.archive.org/web/20140626173432/http://www.chesscafe.com/text/hansen128.pdf

http://www.gambitbooks.com/pdfs/FCO_Fundamental_Chess_Openings.pdf
Another such book is Back to Basics: Openings by FM Carsten Hansen.
http://www.thechessmind.net/blog/2010/1/30/a-brief-review-of-carsten-hansens-back-to-basics-openings.html
From time to time, a chess opening introduction has been included in a general discussion of chess, such as The Mammoth Book of Chess by Graham Burgess.
https://web.archive.org/web/20140708093123/http://www.chesscafe.com/text/review756.pdf