Hi,
The only way to add a file attachment to the signed PDF file without invalidating the signature is to add the file attachment annotation (Add comments → Attach file within Adobe Acrobat). However, Adobe Reader will still report the file as “Signed and all signatures are valid, but with unsigned changes after the last signature.”
If only images are going to be attached and the attached image file doesn’t have to be preserved on a byte level, then the solution from the Placeholder to insert image on PDF - #4 by stipo.gembox could be applied. Image button(s) should be added to the file before signing, and the user can click those buttons to insert images into the signed document. In this case, Adobe Reader won’t report unsigned changes after the last signature.
I do not know the exact details of your workflow, but I assume you want the user to download the PDF file you signed, fill in its form fields, and provide attachments to be attached to the file. I recommend that after the user uploads and submits the filled-in PDF file and attachments, you remove the signature from the file, add attachments to it, sign it again, and return that version of the file to the user.
Regards,
Stipo