You need to select All not just the page you want to edit.
The best practice is to go back to the person sending you the PDF and ask them to send the file in the original format. You should then edit in that program. If it is not Illustrator, then opening the PDF in Illustrator to edit has even more problems than if the PDF was exported from Illustrator from layers getting combined, to paragraphs and words splitting it up. If they do not understand this is the way to work you should charge them extra as they really do not know what they are doing and deserve to be charged extra for the additional efforts needed to edit their submissions.
PDF is not meant for editing, it is meant to be used for a final format eg: post to the web, emailing, Anything besides for fields being filled out or comments added is not the correct way to work.
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