Please refer to linked article on how to check Security Settings of Document: https://kb.foxitsoftware.com/hc/en-us/articles/360040240592-How-to-view-the-security-settings-in-a-document-
If the Security Properties show that Signing is allowed, but the creator of the document has not added a digital signature field on the interactive form and you are unable to add a signature field as shown below:
Check to see that Changing the document is allowed:
If "Changing the Document" is set to "Not Allowed", you will need to contact the original creator of the document and either set them to allow changes to the document or create an interactive signature field.
