As an update - Excalidraw is really good these days and you can handwrite on PDFs on iOS if you just press on the 3 dots in the top right corner, choose share, and then choose Markup.
When you are done, press “done” and “delete PDF”. The PDF is still saved and you have your handwriting.
Though there is a bug when you add other types of pages: Handwriting via Markup on iOS doesn't save if you 'insert page from file' while handwriting
Annoying but at least it can be worked around.