I cannot edit PDF file

There is editing which in PDF parlance means owning a third-party PDF editor to make changes to the original PDF content, and then there is Annotation, where you add a content layer of annotations to the PDF, and the original PDF content remains unchanged. Apple's Preview is not a PDF editor, but rather provides annotation features.

Which are you inferring by the term edit?

It's clear that the problem is in the pdf's author. I think that the author just chose the "not to edit" setting, that's why you cannot do it. My suggestion is the same: open the file in a pdf editor like https://pdfchef.com/merge-pdf.html and then you'll be ab;e to do whatever you want.