When Microsoft announced its plans for the "PDF killer" Metro, a lot of thought was given to the likelihood (or not) of this format becoming a credible alternative to Adobes solidly installed PDF format.
It is true that the announcement, which came in the wake of Adobes merger plans with Macromedia, clearly pitted Microsoft against Adobe—a company that after the proposed merger may be getting uncomfortably big for Microsoft.
Surprisingly, most analysis of the XML-based Metro format completely overlooked two rather crucial aspects of PDF, which have led to its position in the market today.
The first one is portability: PDF stands for "PORTABLE Document Format" and initially the core vision that drove John Warnock to develop PDF was to be able to render documents identically on a number of platforms, including, at the time, DOS, Windows, Unix and Mac OS, and covering today other platforms such as Palm OS and smart phones.
By contrast, Microsoft has not announced any cross-platform support (after all, who cares about anything except Windows?) and hopes third parties will provide cross-platform functionality. (It is not quite clear from Microsofts statements whether Metro will require Longhorn to function.)
The second point is more technical, but every bit as crucial, and it is called font embedding.