No of course not. You can do whatever you want with a PDF that you have obtained legally as long as you keep it to yourself. Where you can get into trouble is if you take copyrighted content and re-use it in a plagiaristic way. Then you may be in violation of copyright law in your locale. Even if you are not technically in violation of copyright law you should still learn how toe up with your own content. Learn how to write things in your own voice even if it is heavily influenced by the writings of others. Learn how to create artwork that you can use in your publications even if someone can tell who your work is styled after. If you are a terrible artist use artwork that is open source or that you have permission to use and attribute the artist. This is why I haven bothered withs for my own document creation. PDFs are meant to electronically distribute work by an author not to give other authors free access to their content so it can be reused without permission.