 <p>Adding my solution as answer, per halfer's advice: Solved this one, because I was passing the content to Zend_Pdf as a string, i should have been using Zend_Pdf::parse($new_pdf);, as it very likely says in the manual. (oops)</p> <p>Further; I solved pretty much ALL of my problems with digitally signing PDFs of various versions and form constituents by moving to TCPDF, as several of the articles here suggest. A similar caveat was met with TCPDF though, when using strings, ensure that you are using TCPDF's 'writeHTMLCell' instead of 'writeHTML'. And watch for PHPs 'magic_quotes', errant whitespace, encoding, and goblins.</p>