PDF/A-1 is based on PDF 1.4 which is a pretty old standard. If your PDF contains PDF features that were introduced in later PDF versions, you cannot convert this PDF to PDF/A-1. It is quite possible that the signing operation uses some advanced features not available in PDF 1.4. For instance, if you sign with a certificate that uses SHA-256 hash, you cannot convert this signed PDF to PDF/A-1 because PDF 1.4 does not support SHA256 hash. This is just an example. Your case might be different but along the same lines.
Are you trying to convert a signed document? This might work but is utterly pointless as the signature would be destroyed. Signing must be the LAST step.
Thanks for your answers.
As written in my initial post, I need to perform the following operations in the order they are listed:
1) convert MS office documents and PDF documents into PDF/A-1b, to report in case of requests of judicial authorities . It doesn't matter if some PDF features are lost in the conversion process
2) digitally sign the newly created PDF documents
As stated, I kindly ask for suggestions about how to do that using Acrobat Pro actions or by means of any other existing tool.
I also had a look at iText and DevExpress that provide frameworks to develop the workflow I need to implement, but I'd rather doing something via Acrobat.
Sorry, I posted the wrong response. How large is "massive"? Acrobat is an interactive tool with some very low volume automation features.
I'm talking about some dozens of documents.
As I mentioned in my previous reply you must use signing features available in PDF 1.4 (Acrobat 5) and not later in order for your PDFs converted to PDF/A-1 to be still PDF/A-1 compliant. If you sign all your PDFs with the same certificate (digital ID) then you can use Actions feature in Acrobat DC to convert all PDFs, say, in the same folder, and sign them. Be aware of which signing you're using so that you do not break PDF/A-1 compliance.