You can integrate AltaMail with the WritePDF app to edit PDF files and add signatures, comments or other details. Install WritePDF here https://esz.nz/wpdfi

When you read an email with a PDF attachment, you can do a long tap on the PDF attachment to see the action menu. Then select "Edit" and the PDF will be opened into WritePDF. You can also select the attachment and tap on the action menu button.
You can do the same process with files in the files screen.

You can then edit or add a signature to the PDF in WritePDF. Then tap on the share button (square with arrow) in the PDF toolbar. Choose "Email as a PDF file" to open the PDF back into AltaMail.
AltaMail will then ask what you want to do with the new file. One of the options is to reply to the email you are viewing and attach the new PDF file.