If you really must print it out for the signature (which seems a bit backwards), you can scan and email it back in to the document library. Have a look in the document library settings/Communications/Incoming Email Settings; test adding a subject to the scan you send, you might be able to put something useful in the subject line to identify what the document is or else to categorise it in some way.
If those are your requirements, then fine. However, I'm currently working on something similar, except I am using forms rather than documents, but I still think it is relevant. You can use an OOTB approval/signature workflow to route documents through to the approvers (people that have to sign). I have used an electronic signature field, Infowise can provide these (there plenty of alternatives though e.g. Adobe have something too); the signature field can be signed using a mouse (not recommended) or a signature pad (Topaz supply something which integrates well with the Infowise field).
Here is a link to a question I'd posted on signatures - have a look at the pictures if you want an idea of how the signature strip works.
You might find browsing through the [Digital Signature] tagged questions useful for your research.