A PDF is really an electronic print of your document & is not intended for future editing. You should always save a copy of the original document in the program that created it & use that for edits. You can then export or print the revised document to PDF, again saving the original.
Open your PDF (Preview app will work) Select All and Copy, then paste into a new Pages document. You can now edit. It probably won't come with all the bells and whistles that your original Pages doc had. But at least you will have editable text.