Is there a free PDF printer / distiller that creates signable documents?

Well, this passage from the PDF 1.7 spec doesn't look too encouraging:

12.8.2.3 UR

The UR transform method shall be used to detect changes to a document that shall invalidate a usage rightssignature, which is referred to from the UR3 entry in the permissions dictionary (see 12.8.4, “Permissions”). Usage rights signatures shall be used to enable additional interactive features that may not available by default in a conforming reader. The signature shall be used to validate that the permissions have been granted by a bonafide granting authority. The transform parameters dictionary (see Table 255) specifies the additional rights that shall be enabled if the signature is valid. If the signature is invalid because the document has been modified in a way that is not permitted or the identity of the signer is not granted the extended permissions, additional rights shall not be granted.

EXAMPLE

Adobe Systems grants permissions to enable additional features in Adobe Reader, using public-key cryptography. It uses certificate authorities to issue public key certificates to document creators with which it has entered into a business relationship. Adobe Reader verifies that the rights-enabling signature uses a certificate from an Adobe-authorized certificate authority. Other conforming readers are free to use this same mechanism for their own purposes.

After coming across this post, I found a free software called Digisigner which has the sole function of signing existing PDFs, whether Reader allows them or not. After that, Reader shows and validates the signature 100%. It's a workaround, but not nearly as complex as yours. It allows both the invisible signatures and visible stamps. There are some catches:

• its renderer being unable to show ligatures, but once signed and reopened it's exactly as it was)
• the big downside for me is that it can't use public-key encryption or set the "signable" bit so that Reader will sign it.