Added PDF tag

free-programming-books.md

@@ -955,7 +955,7 @@ Original Source: [List of freely available programming books](http://web.archive
 ### Delphi / Pascal
 
 * [Essential Pascal Version 1 and 2](http://www.marcocantu.com/epascal/)
-* [Free Pascal: Reference guide](http://ftp.icm.edu.pl/packages/fpc/docs-pdf/ref.pdf)
+* [Free Pascal: Reference guide](http://ftp.icm.edu.pl/packages/fpc/docs-pdf/ref.pdf) (PDF)
 
 
 ### DTrace