editing a pdf on a mac for free, is preview enough?

One thing to know with Preview: it saves your changes straight into the original file. Do File > Duplicate first if you want to keep the clean copy. And if you've used the white box trick, the original text is often still in there underneath and can be selected or copied. File > Print > Save as PDF flattens it, or better, don't use that trick for anything sensitive.

For most things, yes. Preview can fill in forms, add text boxes, highlight, sign, and delete or reorder pages (show the thumbnail sidebar and drag them). You can even merge two PDFs by dragging one into the other's sidebar. What it can't do is change the existing text in the document. If you need to fix a typo in the original wording, you need a proper editor, or cover it with a white box and a text box on top, which is a hack but works for simple stuff.