This error may be caused by the following reasons:
1) The signed document has been forwarded to you by e-mail.
When sending signed documents by e-mail it is recommended to compress the signed file with an archiver and send the compressed file. Failure to do so may lead to damaging the –ddoc file, making later opening of the file impossible.
2) The signed document is opened directly from the mailbox where the contents of the file are scanned by an antivirus program before its opening/saving.
The file contents may change after scanning, making the opening of the file impossible. To avoid damage you may try to temporarily deactivate the antivirus program and opening/saving the .ddoc file when the antivirus program is not active.
If some of the above described reasons did not solve the issue then please let us know by sending an e-mail to firstname.lastname@example.org.