i am generating document with attachment when i select an attachment when i select the delivery method and click on run i get the follwing error

 One or more of the input documents is password-protected

Now the other document which I am selecting as attachment is not password protected then why I am getting this error, just to give you info on this attachment I am generating this attachment from getting bas64 data out of our different service and then creating file with that base64 data.

now i am able to preview this file in salesforce only when i select it as attachment during  docgen document geneartion process i get the error.

Also if i generate document without this attachment it works fine.

My question.

1-why i am getting this error even though there is no lock on file.

2-if the pdf which i am using as attachment is too complicated will i get this error

3- is there a way to resolve this

Thanks