It is open. The reason why Adobe threatened to sue Microsoft seems to be that they would use their monopoly situation to force a certain Adobe product line out of business by offering the "save as PDF" in Office.
Believe it or not, but a lot of companies actually pay money for propriety software used only to convert their documents to PDF format. The lawsuit threat proves it.
The monopoly situation is what allows them to sue. If everyone was using OpenOffice I suppose they could sue Sun(?) for making Adobes business idea irrelevant unless Sun removed the PDF conversion.
So Microsoft actually wanted to do something good and implement an open standard, even if it was probably not out of love for open standards in general. They were stopped by anti-monopoly laws that should be a good idea since monopolies are bad, but which in this case is used to sustain Adobes sales of the same function a few more years just because the customers they would otherwise loose doesn't realize they could get the same thing for free. Their terms for letting Microsoft include the function was that they should charge extra for it and let Adobe in on the revenue. Adobe is just after the money whether it comes from their own products or from getting a piece of the monopoly.
You may consider it good or bad, but the fact is that monopoly regulations doesn't let the market leader compete on even terms featurewise with other products when it comes to functionality that can be seen as a separate business niche.
Now I am sure that Adobes package does a lot more than just convert other documents to PDF and some customers actually need that extra functionality, but Adobe still considers the conversion alone important enough for their business model to go to court over so that alone is the important feature here.