Altering the Content of a PDF?

I download a PDF File--Perhaps its a Book; in my own case, it is often a Patent.

In a Book, I would often like to enter my own marginal notes and comments--much the same as I often to with Real Books--or add highlights.

In the case of Patents--Some of them require lengthy study to understand--and often I'm trying to take the design one step farther...

I would really like to be able to freely insert whole sentences--even paragraphs of my own interjections--along with my own drawings--or judicious modifications to the original--like arrows--or names...

My purpose is not to falsify and then Publish--but simply to have free use of the Document.

Can some kind person tell me how to do this--In a way that a Kumpooter Eelitrate like me can follow?