I keep returning to the concept that software provides an environment for creativity, and that these tools create a context for the content we create within them. What we publish is shaped by the way we publish it.
The canonical example is Microsoft PowerPoint, which is (often wrongly) accused of crapifying anything that's entered into the slideshow program. But I've been thinking a bit about Adobe Acrobat, especially since very little of the content published in Acrobat's PDF format is actually created with the tool. I'd suspect the overwhelming majority of PDFs are created in Word or a similar tool and then output in the format. But there seem to be conventions that have developed among doucments created for PDF output, even if different tools were used to author them.
The PDFs which prompted this most recent rumination are, for the most part, excellent. Mark Hurst's Uncle Mark 2004 Gift Guide & Almanac, Mark Pilgrim's Atom API presentation, the Pew Internet Project's Internet consumer report, and the Ad Age Marketing 50 are all well-written, well-designed documents. This makes sense, as choosing the PDF over valid XHTML only makes sense if exact positioning and detailed formatting are the most important concerns for the presentation of a document.
But none of these documents use any features exclusive to PDF that would preclude them being published as HTML. Indeed, features such as the affiliate links in Uncle Mark's guide and the URLs referenced in Mark Pilgrim's API presentation would work better in a browser context than they do in the Acrobat reader. So why PDF? Especially given that the harsh criticisms levelled at the format are largely true. (Granted, these problems are due to Adobe's shoddy implementation of the reader experience while web browsing, but the difference is largely academic since the format can't be separated from the reader used to view 99% of the documents in the format.)
It seems that the PDF format signifies something now, and it's something more than just user inconvenience. In addition to requiring the user to shift mental modes, ("I'm seeing something designed as a PDF now, this must be serious information...") the requirement that a document either be downloaded or viewed in a context that's radically different from standard web pages seems like a subtle assertion of authority by a document's creator. The decision to switch from standard HTML to PDF isn't arbitrary, but it isn't based on technical requirements either. It's based on the value that an author wants to assign to the work, and it benefits from the still-prevalent, though rapidly fading, consensus that print work is somehow more inherently valuable and authoritative than web pages and other online content.
This is evidenced in several ways. Documents which are offered up for a fee are frequently in PDF format, though for unprotected documents there's no reason the content can't be presented as HTML. And even password-protected PDF documents rarely make use of any of the advanced features which theoretically distinguish PDF from HTML. If the goal is to preserve formatting fidelity for the user while providing a good user experience, Macromedia's FlashPaper offers a much more pleasant in-browser experience that doesn't require the document viewer to take over the entire window and chrome from the standard browser toolbar. So the PDF decision is entirely about communicating intent.
The implications of the space which PDF has marked out in the content arena are very interesting. Without support for anything like HTML's IFRAME tag, it's not easy to insert dynamic content into a PDF unless the document is generated at the time it's downloaded, instead of being a static file that can be emailed around or passed along. Since Overture's context-sensitive text ads and Google's AdWords program have redefined so much of the financial model for content that's provided online, this presents a significant impediment to PDF's ongoing dominance of the market for profitable text content online. And FlashPaper's ability to exist within the larger framework of an HTML document means that publishers interested in augmenting a print-like publication format with text ads may well choose to use the newer format, especially given that the Flash plugin has a broader reach than the Acrobat reader and is less resource-intensive.
Going forward, it will be key for the PDF format to embrace users' new expectation of simple publishing functionality, which has become a baseline expectation with the proliferation of weblogs and other lightweight publishing systems. But the most interesting area to watch going forward will be watching whether PDFs remain the format that authors use to communicate seriousness of purpose or professionalism.
In the weblog realm, we already see perception of content influenced by the tools used to publish it, and with the ability to generate well-formatted, device-independent documents through simple tools publishing standard CSS and XHTML finally reaching maturity, it's possible we'll see a return to traditional markup and the relatively rich experience of contemporary web browsers being chosen as the preferred medium for publishing "serious" information.