Well all of the comics on the site are believed to be in the public domain but some of the characters in them may still have trademark restrictions on what you can do with them.
An example of a character with restrictions would be Captain Marvel which was originally published by Fawcett in the 1940s and 50s. While many of those stories are PD, DC Comics currently has trademarks on the character (as well as the term "Shazam") which would prohibit anyone else from doing new stories featuring the character. To make it more confusing Marvel Comics owns the trademark to the name Captain Marvel which restricts anyone else (even DC) from using the name in any promotional way (pretty much keeping the name off covers and advertising in DC's comics featuring the original Captain Marvel).
So you could technically reprint the PD Captain Marvel stories but would be restricted in using the Captain Marvel name (or Shazam) in your promoting of the reprints or on the cover of the reprint.
Hope this helps a bit.