Adobe's PDF has become the ubiquitous standard for storing and distributing official documents. Although I am no great fan of Adobe for mostly unrelated reasons, there are a number of good reasons for this: it is highly portable (indeed, PDF is short for Portable Document Format), with readers available for almost every operating system that can handle a graphical display, it displays exactly on a screen as it would if you printed a hard copy, the compression is decent, and it supports a variety of nice features such as hyperlinks and embedded forms.
It also supports a number of features designed to allow the document creator to hinder the abilities and rights of the end-user, namely flags to ask PDF reader software to disallow printing, copying, alteration, or the appending of notes, even in the small portions covered by fair use. Tools to edit a PDF file after creation are also nowhere near as portable as the readers, and indeed are quite hard to find. This makes it even more attractive to certain types of people, but fortunately isn't anywhere near as bad as it could be (and might even be a good thing overall — more on that in a bit).
The quality of implementation varies; Adobe's own reader (and particularly, browser plugins) for MS Windows have a history of being buggy and unstable, as well as slow to load, leading one blogger to rant about how difficult it can be to read government documents in this format, and accusing the government of having effectively set up an "Adobe monopoly". This rage is somewhat misplaced; the PDF specifications are freely available (albeit in PDF format themselves), and have been used to build competing viewers and other products, some of which are even Free Software, such as XPDF. There are alternatives to the Adobe reader (though admittedly, using them may require you to move away from MS Windows as an operating system).
The format isn't popular because it is government mandated; it's popular because there really aren't any competing formats. The aforementioned blogger's rant notwithstanding, HTML isn't really an option. For one, HTML won't preserve the likeness to an original printed document once it is converted to electronic form, and thus it won't allow identical reproduction of that document elsewhere. For another, almost nobody gets HTML right. Try this for an experiment: go to the W3C Validator, type in an arbitrary web page, and see if it passes. Most pages are broken beyond belief, either because the author has no idea how to write HTML, or because he or she is using a tool that doesn't generate correct results. Even my own site, powered by the Free Software CMS system Drupal doesn't pass completely cleanly (though to be fair, the errors are relatively minor, and it uses XHTML, which is a much harder standard to write to than plain HTML). It's a testament to modern browser technology that as much of the web is as usable as it is.
Back to the point, however, I'm not entirely unhappy about the emergence of PDF as a document standard, partly because something was needed to do what it does, and partly because the parts that I don't like about it are so badly designed that they can be ignored. Since it is an open standard, there's nothing preventing implementations from completely ignoring the restriction flags, which can be a very handy thing when paranoid but technnology-ignorant lawyers try to lock down a document (though Ed Felten has a somewhat less charitable view of their motives for doing so). Since XPDF is Free Software, it can be easily modified to disregard those flags (and in fact, the version that Debian GNU/Linux ships disregards them by default, unless you specifically choose to have the software start honoring them again).
This doesn't mean that you don't have a right to be upset when you end up saddled with software that sucks too badly to let you read official documents without taking out your system; it's just that neither PDF as a format nor the US government are to blame this time. Adobe is certanly a fair target — but only for making software products that suck.
