what's the free way to edit a pdf on a mac

Preview, which is already on the Mac, does more than people expect: text boxes, highlighting, signatures, deleting and reordering pages, even merging PDFs by dragging thumbnails from one into the sidebar of another. What it can't do is change the existing text. For that, LibreOffice Draw opens PDFs and lets you edit the text, just expect the layout to shift a bit on complicated documents.