Recently, cinamod tacked on a rant about PDF being a non-free format. I'd like to hear more about that. Certainly, my views are probably coloured by working on Ghostscript but I also think I care more about free formats than post people, and it's not really my perception that PDF is evil.
It's true that the format is controlled by Adobe and they don't have an open development process. On the other hand, the is good, freely available documentation of the format and Adobe has generally behaved in a way consistent with their being congnizant of it's value as an interchange format with multiple vendor support. Both of these things contrast with Microsoft Office document formats, the other example under discussion.
It is true that Adobe claims some patents on aspects of PDF. Their is however a blanket grant for applications compliant with the spec.
It is also true that the PDF spec, especially later versions includes a number of non-free (or silly) formats by reference, like JBIG2 and JPEG 2000 (the later is at least getting lower risk as time goes on). The latest release (PDF 1.6) even includes support for embedding the U3D format, which as near as we can tell hasn't been published in any form yet!
But, I don't actually see problems with the issues cinamod mentions directly. The LZW patent expired in the last jurisdiction a year ago. The Compression Labs patent on baseline JPEG (if that's what was being referred to) is generally considered invalid. The colourspace conversion issues are covered by the grant mentioned above.
Better, one advantage we have in Free and Open Source software is that we can choose the features we implement based on technical merit rather than the need to sell this year's upgrade to our software. This is I think were PDF really starts to look good. It's much easier to write a parser for than PostScript, which is/was the old standard, and the updated imaging model, compression, and portable rendering features make it a much better choice for everything we traditionally used postscript (which was another Adobe-controlled standard) for. The only drawback really is that you can't generate simple files with printf() like you can with postscript.
There's no compelling argument for us to be producing documents containing JBIG2 or JPEG 2000 images. The spec does contain support for LZW, but for the most part we continue to do what Ghostscript did until last year, and that's only support compressing with the free Deflate (zlib) filter.
So what exactly is non-free about it? It's easier to get the documentation and start implementing than many "more" free formats, like baseline JPEG or TIFF fax compression. By producing files based on a sane subset we can avoid both the patent and technical excesses and parent format. In fact, a number of such subsets are already defined and available for reference, like PDF/X or PDF/A.
Of course it's nice to have something controlled by and open community (like the Xiph multimedia codecs) but network effects are also very important. I think it's a good idea to just use PDF and make it our own.