how to edit a file someone else created

While it's technically possible to edit PDF files directly using Adobe Acrobat Pro, describing it as cumbersome and problematic would be charitable at best.

You can, however, with reader-enabled PDF files apply suggested edits over the top of your PDF content — strikethroughs, sticky notes with suggested additions/edits, highlighting tools and drawing tools — which would let you tell your book editor what you want done with your book layouts. Additionally, printing it with your markups would give you a hard-copy record to accompany the digital trail you create marking up the PDFs with your desired edits.

However, your editor will have to provide PDFs which enable these features for you to use for markup/editing purposes. And I would strongly recommend relying on your editor to apply those edits to the original InDesign files, then having your editor provide a set of revised PDF files to confirm the edits have been made. Proof the revised PDFs carefully to ensure you're happy with the results.

There is no third-party application which will allow you to make edits directly to the InDesign files. You'll need Adobe InDesign to do that. And as I wrote earlier, editing PDFs directly is fraught with peril. Providing markups, and having the editors provide revised PDFs to confirm changes are made, is a proven and widely-accepted commercial workflow.

I know this isn't the answer you're looking for, but I hope this helps.

Randy

PDF can be edited by Adobe Acrobat DC
Here is Help: https://helpx.adobe.com/support/acrobat.html
You can get a trial version and then decide if you need it more.

If you have a working InDesign file, you also can use a trial and help. starting from Adobe.com and your localization.