Add PDF indications

free-programming-books.md

@@ -329,9 +329,9 @@
 #### Database
 
 * [Big Data Now: Current Perspectives from O'Reilly Radar](http://shop.oreilly.com/product/0636920022640.do)
-* [Database Explorations](http://www.dcs.warwick.ac.uk/~hugh/TTM/Database-Explorations-revision-2.pdf)
+* [Database Explorations](http://www.dcs.warwick.ac.uk/~hugh/TTM/Database-Explorations-revision-2.pdf) (PDF)
 * [Database Fundamentals](http://public.dhe.ibm.com/software/dw/db2/express-c/wiki/Database_fundamentals.pdf) (PDF)
-* [Databases, Types, and The Relational Model: The Third Manifesto](http://www.dcs.warwick.ac.uk/~hugh/TTM/DTATRM.pdf)
+* [Databases, Types, and The Relational Model: The Third Manifesto](http://www.dcs.warwick.ac.uk/~hugh/TTM/DTATRM.pdf) (PDF)
 * [Foundations of Databases](http://webdam.inria.fr/Alice/)
 * [Readings in Database Systems, 5th Ed.](http://www.redbook.io)
 * [Temporal Database Management](http://people.cs.aau.dk/~csj/Thesis/) - Christian S. Jensen