It seems to me that there is something more basic going on in the way IOS creates and renderd pdfs. I do have an app designed specifically for marking up pdfs.. and still have this issue.
I can save a pdf (from Outlook for ios) to it.. make annotations. those annotations are visible in the app. but
When I "open in another app" to send the file.. the annotations are gone.
When I open the file in OneNote.. and render it as a printout.. the annotations are gone, although when I then click perview.. the file opens and the annotations are visible.
When I look at the same email in Windows - the attachment opens in its windows counterpart.. and the annotations are visible.
Also - in windows - I cannot take that same pdf file in OneNote and render a printout - it's as if the file is corrupt.
To me - this seems like ios is saving the file with an associated graphic overlay - and then in ios it is failing to render that overlay.. and possible worse.. is not doing it in a way that other apps (like MS OneNote) can recognise; it should be possible to save a flattened copy - where the overlay is 'printed' irrevocably on the pdf.
For a lot of usage cases - this is the only way this functionality can be trusted - what is the point of signing a document if that signature is only an associated graphic that can easily be changed?