Changing .pdf to .pages

Your best bet is to edit the source document from which the PDF was created and then regenerate the PDF.

PDFs are end stage documents intended for sharing or printing. They are not word processing documents. You will need a paid PDF editor to alter the original content of the PDF as Apple's Preview or Adobe's Acrobat Readers are not editors. Pages cannot open PDFs for editing but one can drag and drop a PDF page into Pages as a final (uneditable) document object.

If you have access to MS Word v16.31 or later, you can use it to open the PDF and it will then convert it to a Word .docx document. You can make changes to the .docx content in Word and then File menu : Save As : PDF.

The free trials of any paid PDF Editors will watermark the original PDF, or restrict certain features, so that isn't an option. Copy/pasting the PDF contents to a word processing application (e.g. Pages) will not necessarilty retain the original text flow or formatting, so the potential for substantial additional work to get the word processing document to appear as the original PDF before exporting.

I still believe that you should put that PDF on a USB stick, and see if a friend that has a current version of MS Word, would allow you to convert that PDF to a Word document, make your changes, save as PDF, and then put the Word document and PDF back on your USB stick. Or, you make a copy of the PDF and find someone that has a current version of Adobe Acrobat Pro (not Reader) that would allow you to edit that PDF.

The second paragraph was the last stop before you would need to purchase a proper PDF editor and outside of Code Industries Master PDF Editor (single-purchase, ~$70 USD), they are all more expensive, though in the Mac App Store (e.g. PDF Editor, Nitro PDF Pro).