It's not easy finding a truly free PDF editor that lets you not only edit the text in the PDF but also add your own text, change images or add your own graphics, sign your name, fill out forms, etc. However, below is just that: a mix of the best free PDF editors that include all of those features and more.
Some of these are online PDF editors that work right in your web browser so all you have to do is upload your PDF file to the website, make the changes you want, and then save it back to your computer. That's the quick way, but often times an online editor isn't as fully featured as its desktop counterpart, which usually has a much wider range of abilities.
Since not all of these free PDF editors support the same features, and some are restricted in what you can do, remember that you can process the same PDF in more than one tool. For example, use one to edit the PDF text (if that's supported) and then put the same PDF through a different editor to do something supported in that program, like to edit a form, update an image, or remove a page.