iText: what type of certificates do people use to

I have a low volume (<500 PDFs/year) application for automated digital-signing of PDF files using iText in Java on Linux.

I've got iText adding a digital signature to PDFs using my SSL certificate. Is this a valid method to prove the PDF was generated by my domain (e.g. server)? Can it be used somehow to get the green checkmarks showing "trust" in Adobe Reader?

If not, I should use a certificate intended for PDFs (e.g. not my SSL certificate), so that the little green checkmarks indicating "trust" appear naturally when the user opens the PDF document.

The book http://itextpdf.com/book/digitalsignatures does a great job introducing this topic to me (I have very little experience in this area).

The book talks about a SafeNet Luna device (an HSM), but's it is much too expensive. I only require a minimal solution, and the Luna has a lot of bells and whistles. The Luna PCIe device is less expensive, but I don't require any features other than providing a certificate I can use to sign. Also, the USB-based SafeNet iKey device seems only to be marketed to Windows devices. Has anyone got iKey working with Linux? Is it even possible? Do other companies offer USB based devices that work on Linux?

I'm looking for a minimal solution for serving automated digitally signed PDFs on a Linux box. I'm sure a lot of small businesses have similar needs. I'm just trying to tap into existing knowledge out there. How do people solve this problem?

The solutions I see for automating this process assume large corporations using Adobe Live Cycle, and priced accordingly (see for example: https://www.globalsign.com/pdf-signing/compare-pdf-signing.html). But small businesses need to automate things too.

Ideally someone would sell a certificate similar to the SSL certificates, but for PDF files. Is there such a thing?

Is hardware (of some sort) a requirement (seems so)? If hardware is a requirement, are there any minimal solutions out there (e.g. with limited functionality other than enabling digital signing)?

Hoping someone can help me see the forest from the trees. What's the conventional wisdom?

Regarding signing with your SSL certificate: in a future iText version, we make require that the key-usage of the certificate indicates that the certificate can be used for non-repudiation. For now, we make checking the key-usage the responsibility for the developer, but in a perfect world, you should only sign with certificates suited for non-repudiation, and your SSL certificate probably doesn't allow this.

Regarding the green check mark: unless you can ask the consumers of your PDFs to add the root certificate of your certificate to the list of trusted identities, you'll always need a public/private key stored on hardware to get a green check mark.

Regarding the price of an HSM / USB key. USB keys are much cheaper, but usually they are meant for manual use (usually they have a limit of signing only once every second). I think that GlobalSign has a flavor of keys that work on Linux. As for HSMs, one of our customers told us that he bought one from Utimaco because it was less expensive (but I don't know what budget he had or spent).

No price info, but maybe a good read for inspiration: http://www.opendnssec.org/wp-content/uploads/2011/01/A-Review-of-Hardware-Security-Modules-Fall-2010.pdf