PDF is a fantastic format, its portable, readable, widely adopted and it is way too tightly linked to physical documents. Most American PDFs are produced on a Letter sized template, a paper-sized used exclusively in the US and which is both expensive and find to hard in the UK. Despite some good efforts by the Adobe people to try and get a Letter document to stretch to other formats the truth is that printing out a Letter document onto a A4 results in a less than satisfactory print that just looks weird. Trying to print a document to something like A5 for quick reading or to get a booklet version of the document is even worse.
For any e-publisher who wants to sell to customers outside the US please provide an A4 version of your product!
And PDF software providers, please stop putting so much effort into locking a PDF into a read-only format and start looking at a rendering that is independent of the paper size.