Mail app won't open when sending signed documents

I am not certain I understand the failure you are encountering, but if you are trying to open/fill an attached file from Exchange you need to make sure that it is one of the supported file types. Fill & Sign iOS supports PDF and image (.png and .jpg) file types. If this is not the issue you are facing can you please provide more detail about the workflow you are attempting and I'll see if I can assist you further.

Andrew