I can’t use Acrobat Reader to « modify the pdf »

Adobe Reader doesn't modify PDFs, it would allow Fill & Sign, comment, save a copy and print, as long this is not restricted by the source PDF document that you're viewing with Adobe Reader.

To edit and modify a PDF use your Adobe Acrobat Pro DC (paid subscription). If you don't have Acrobat Pro DC installed in your computer you need to download and install it.

You can go here: https://account.adobe.com/plans/

And check under "Manage Plans" if you have Acrobat Pro listed there (if your subscription is current it should appear listed there). Click on the download button and install it.