improving opening

hey guys i would like to improve my opening knowledge ,cause i'm familiar with king pawn games and sicilian ,but i often lose against d4 ,are there some free pdf courses about opening theory ?

Have you looked at the Book Openings option under the "Learn" section here?

pick an opening and try it out to see how you like it. find one or two to stick with and learn them as well as you can. mine for black vs d4 are the chigorin and the grunfeld. The Book Openings and Game Explorer sections are helpful for this.

Learn principles.  Fine's Ideas Behing the Chess Openings and School of Future Champions 2: Opening Secrets should shed some light for you. 

Basically, in the opening you have three main opening tasks: develop minor pieces, castle, and connecting the rooks by moving the queen.  These don't have to be in order and also keep special cases (usually within the context of a specific plan and/or preperation) in mind.  An example of when it seems acceptable to break principles within the context of a plan from Questions of Modern Chess Theory (one of Fischer's favorites, one of the reasons he taught himself Russian as there wasn't an English version available in those days)

Also find a chess hero to model your openings after.  I'd recommend Boris Gelfand for now since he's modern enough, solid, and has an amazing sense of danger.