Researchers from Ruhr-University Bochum in Germany have broken digital signatures on PDFs and managed to create fake signatures on 21 of 22 viewer apps and five out of the seven online PDF digital signing services. This archive contains both whitepapers produced by the university.
You can download it from the following link: https://packetstormsecurity.com/files/download/151855/pdf_signatures.tgz
Source: https://packetstormsecurity.com/files/151855/PDF-Viewer-Signatures-Broken.html