There's a ton of PDF forms that I deal with at work that only work with Adobe Acrobat. When I open them in other PDF viewers, it'll be a single page telling me that if the PDF doesn't load in a few seconds then my PDF viewer is not compatible with the document.

I'm not an expert, but I imagine that they were created with Acrobat and it embeds some (proprietary) stuff in the PDF that only works with Acrobat. Acrobat is also the only PDF client that I'm aware of that supports signing PDF forms with certificates. There's other features in some PDFs that I've encountered that'll only work in Acrobat as well, like buttons embedded in the PDF to email the form or dropdowns in the form that don't work in other viewers.

It seems pretty swampy to me when 90% of the PDF forms I deal with can only be (fully) completed in Acrobat.