I have told you about e-Sword before, a very helpful (and FREE) Bible study software tool. I was searching today for an A.W. Pink STEP module for e-Sword and stumbled across a treasure trove! David Cox, a missionary in Mexico, has an online religious library (here) with hundreds of books, articles, and sermons in PDF format. (These are all public domain works so there are no copyright issues.) He even has the works categorized so you can search by topic or by author. You can view the works online, download them, or print them.
One disclaimer to be noted is that the site does contain works that may not be considered biblically sound, but many of those he has included for his own reference. As with any theological book even by "sound" authors, he urges the reader to compare what is written with the Word of God.
I hope you enjoy this cool reading tool!