Abbreviations, acronyms, and style guides

I spent some time last night quizzing myself on the AP Style Guide, a topic on which I have never specifically studied but about which I am curious. My scores varied between the tests I tried. I certainly learned a thing or two. It got me thinking: to what style guide does the Guardian, the newspaper I read most regularly, adhere? I found in a Google Search that they have a series of articles in which they outline their style guide, divided into individual articles for each letter of the alphabet ^{1 2}.

I opened up the first guide on the letter “A”. Without any particular goal in mind, I scrolled through the page and saw there was a section on abbreviations and acronyms. A quick glance uncovered the answer to a question that has popped into my mind occasionally when reading journalism from some publications: why are acronyms (i.e. NASA) spelled out in lowercase?

the Guardian style guide has codified a rule for their publication on this topic:

Use all capitals if an abbreviation is pronounced as the individual letters (an initialism): BBC, CEO, US, VAT, etc; if it is an acronym (pronounced as a word) spell out with initial capital, eg Nasa, Nato, Unicef, unless it can be considered to have entered the language as an everyday word, such as awol, laser and, more recently, asbo, pin number and sim card. Note that pdf and plc are lowercase.

This directly answers my question, and also raises another one: why should PDF be in lowercase? I see the ISO standard for the underlying format doesn’t use capital letters in the title ³.