I've encountered .pdf files which internally embed a proprietary Adobe extension called XFA[1]. I think they are created using Adobe's LiveCycle product. They

I've encountered .pdf files which internally embed a proprietary Adobe extension called XFA[1]. I think they are created using Adobe's LiveCycle product. They are a real pain because they render fine in Adobe Acrobat, but most other PDF renderers (including browser built-in ones) can't render them. Instead they render a blob of interstitial "loading..." text that is also embedded in the PDF (which the XFA rendering would then overwrite). It was a pain to me personally because I had to figure out a way to do programmatic form-filling of some fillable form XFAs, and most PDF libraries don't work with them (they expect traditional AcroForms fillable forms). But in reading the XFA specification I found it interesting it had its own JavaScript interpreter (including supporting XHR requests as part of some internet-integrated form-filling feature) and another proprietary scripting language called FormCalc. I guess it opened my eyes to PDFs being a container format and the kinds of things they allow you to embed. [1]: https://en.wikipedia.org/wiki/XFA https://en.wikipedia.org/wiki/XFA