Need to massively convert to PDF and sign documents

Dear Sirs

I need to automate a process to be given to company non-techie users in order to:

- convert a bunch of files to PDF/A-1b format

- digitally sign the created PDFs

The aim is to give them a one-click procedure.

I succeeded in creating an action to convert an entire source folder containing several kind of docs (word, excels, images) and save the newly created PDFs in a destination folder.

I made several tests to digital sign those documents, but the best I was able to do is to insert the "Certificates" tool into the action. The thing is I need to insert the password for any document and  the process gives errors.

I'm attaching some images to show the various steps.

1) this is the newly created action

2) When I run the action, I'm asked to provide the certificate and its password

3) the document looks signed, but I need to click on the "Proceed" label in the bottom right

4) then this error appears. Looks like the signed document cannot be converted to PDF/A

Please advise

Thanks a lot

stefano