[mohaine]

I think the format itself could be ok, but not in its current format.

The PDF header can be anywhere in the document. This makes parsing for bad content harder. (Also, you can have a valid pdf that is also a valid zip with the contents being the original file. How do you virus scan this?)

To many old image formats allowed, with readers only supporting a subset. Tiff alone has a bunch of options that are often broken. What happens when you put a multipage tiff in a pdf? (I think you just see the first page in Reader but some other reader might allow you browse them)

Lots of features in later versions that are not well supported. (forms, document libraries, scripting?)

It has been while since I left the document area but while I liked simple PDFs, once you say you support them, you have to support all of them which is almost impossible to do correctly. The later specs just have too many features that are almost unused but add a lot of complexity that really isn't needed in a portable document format. A stripped down/cleaned up version of the spec would be nice.