Portable Document Format (PDF) is a standard electronic document format developed by a group of sociopaths in a psych ward euphemistically known as Adobe Systems Inc. With great forsight, the designers saw the need for a new, more portable document format following the precipitous decline in popularity of Microsoft Office. After intensive market research, they determined that as much as 0.2% of the world's population are unable to open .DOC or .HTML files. Detractors later pointed out that most of those users were either Amish or under the age of 3. Nevertheless, the dream of the universal document format was born.
As the name implies, the key benefit of PDF is portability. Simply put, this means you can publish your document once, in a single format, and annoy your readers everywhere, no matter what platform they're running. Undoubtedly, this concept was inspired by Java's Write Once, Debug Everywhere technology. This design makes PDFs particularly suitable for sharing documents on the Internet. Most internet users are enthralled to interrupt their browsing to download a huge file and then wait for the bloated reader to load (see #Plugin Architecture). This feature is particularly loved by users who routinely open large reference documents to view only a few pages.
One of the risks of this level of portability is that uniform user experience can be degraded. This risk is greatly mitigated by the fact that no one but Adobe actually understands the PDF standard, thereby ensuring that any ne'er-do-wells who attempt to release compatible readers are doomed to failure. Users may be temporarily lured away by the promise of a fast, lightweight, open-source reader, but they will inevitably return when they find their documents look like they were rendered by a crayon-wielding 3 year old with Parkinson's Disease.
edit Adobe Acrobat
No discussion of PDF is complete without talking about Adobe Acrobat. Acrobat is the standard for creating and viewing PDFs, and comes in two versions: Free and Prophylactic (often abbreviated as "Acrobat Pro"). It is rumored that the Pro version can create PDFs, although I've never met anyone who has it, nor have I met anyone who would want to. The thought of paying for this software and having to look oneself in the mirror every morning is simply too unbearable for most users. Hence everyone runs the Free version, which in addition to being a full featured viewer, is also an efficient Adobe marketing material delivery platform.
edit Plugin Architecture
One of the great joys of Acrobat is staring at the splash screen waiting for countless plugins to load (see ADHD). To the uninitiated, it would seem like a simple document viewer doesn't need "plugins". It really needs only one feature: viewing documents. No one knows what all these plugins do, and in all likelihood no one cares.
But this line of thinking is based on a silly and outdated understanding of plugins. People used to think plugins were simply extra, optional features. These people may even be tempted to clear out their Adobe plugins folder to make it load faster (see OCD). And these people will be left behind because Adobe has introduced a new, richer, more inclusive definition of "plugin". Today "plugin" simply means required functionality, such as keyword search or clickable hyperlinks. This major advancement offers the unexpected benefit of performing behavior modification on uncooperative users who really shouldn't be meddling in their program folders anyway.
edit Version History
Arguably the least well understood aspect of Acrobat is why there are "updates". There have not been any new features added or needed since at least version 5. Adobe claims they are "security updates". But it is not clear why opening static, human-readable documents, which should never contain executable code, would pose any security risk whatsoever.
Regardless, users can rest assured that Acrobat comes with its own update service. It is critical to have the service running at all times so that huge updates can be installed without warning the very instant they become available. This is ideal for the average user who does absolutely nothing with their computer except stare at it blankly 12 hours a day.