Quite right, PDF is proprietary, as is GIF for image files. Alternatives
to GIF have been developed for that reason. One danger with proprietary
formats is that they can be manipulated to force behaviors, such as what
Microsoft tried to do with Java. I can import Word files into my (now
open software) Word Perfect, but Microsoft can break it by changing the
code of Word documents. An international standard, on the other hand,
is always usable.
PDF files contain images, but then one can include images with other for-
mats as well. For example, if I send you a ZIPed file (InfoZip, of course,
which is not proprietary) of a HTML page and a JPG (non-propietary) file,
all do is unzip, and run the HTML file, and you see the image linked from
it. There are better alternatives emerging, but I don't see the advantage
of PDF in this regard. While the image in this case must be first conver-
ted into a plain ASCII text, that's true of PDF also.