you cannot do much. Maybe even nothing at all.
This is all in the responsibility of the application that opens the PDF.
Your collaborator should ask you to make the changes.
That would be option number one and the best.
Next best thing: The PDF could be opened in Acrobat Pro DC and some edits can be made there.
If you want edit text all the needed fonts must be installed and available.
And that could also be the cause why Illustrator is converting the text to outlines.
( ACP )
Thank you very much ... Not necessarily PDF
Are there any other solutions regarding the design of editing ID in AI
InDesign to PDF is a final file and should not be opened in ANY other programme.
That is, Illustrator is not a Multi-Purpose PDF Editor. You should only open PDFs in Illlustrator that were created in Illustrator with the Edit Capabilities enabled.
If your client wants to edit in Illustrator then create it in Illustrator and send them an Illustrator file!
You could get most of the layout/text over to Illustrator by using the Creative Cloud Libraries.