2) docx format is wrapped in a zip file, so there is built-in default compression; PDF is not compressed by default
3) does it matter? Most documents I see these days are <1 MB; assuming that universal compression ratio of 3 is achieved, the PDF file would be 3 MB large - not very significant in absolute numbers.
I'd say it's not a conspiracy, just a different format - and that the size difference is not very significant. (Now, a 200 MB docx versus a 600 MB pdf might be significant; but if you are routinely handling 200 MB documents, I'd suspect Something Is Horribly Wrong anyway).
Obviously one of these document types routinely uses compression. If the user composes prose carefully, and avoids repetition, the contrast you cite may be much less.
The gold standard for compressible text is political speeches -- they tend to say almost nothing with as many words as possible. I've always thought part of the decision process leading to voting for someone should include an attempt to compress his speeches.
I loved your analogy in political speeches. I actually have a friend who tracks politician (on www.mumbaivotes.com) - I might actually share this idea of your of compressing political speeches to identify the uniqueness in what is said in the speech ;-)
To me the advantage of a PDF over a Word document is that the former is now open-source. And it is universally regarded as the "correct" format for technical writing.
> ... I might actually share this idea of your of compressing political speeches to identify the uniqueness in what is said in the speech ;-)
The intriguing part is that it's an objective measure, in a field that's called "political science" but isn't remotely scientific. I'd like to see a thorough study comparing political speeches in this easily conducted, almost automatic way.
Another, perhaps more difficult, study would analyze advertising copy for use of words that have no objective meaning, words that appeal only to emotion. My favorite advertising word is "zesty", a frequently-used word with no information content whatever.
In fact, it was Adobe that tried to prevent you from saving to PDF. It threatened Microsoft with legal action (especially in Europe) for trying to offer the "open" PDF format in Office 2007. This was widely reported at the time, but see, for example:
http://betanews.com/2006/06/02/adobe-to-sue-microsoft-for-pd...
Cost quote from them in 2009/Early 2010: 1K to 2K per user depending on features.
I passed.