[Tagbert]

Its really more of a limitation of PDF. It is intended as a static format based on layouts the size of a full sheet of paper. Its internal representation does not lend its self to layout changes. Most attempt to fit it to a smaller screen are problematic in one way or another.

It would be better to start with a document format that was always intended to be flexible, not fossilized.

[Brakenshire]

You make a fair point, but on the other hand many pdfs have massive margins, or multiple columns, or can have text reflowed in a reasonable way. You can't use pdf readers for all circumstances, but with good software you could handle 80-90% of cases.