Hi Ga,
We apologize for the delay in response to your query.
How you want that pdf document to be filled or completed that totally depends on your choice. However, as we know that markup tool will only help you adding text (not filling the text in fillable form) to the pdf and Sketch, zoom in, or add your signature to photos or PDFs in the Mail app.
It wont let you to fill the pdf form as you can in Adobe Reader mobile app. You can test the same at your end by emailing a form to urself and try filling it out.
How it actually works, you can check here- Use Markup in Mail on your iPhone, iPad, or iPod touch - Apple Support
Hope this answers your query.
Regards,
Akanchha