Well, I don’t know about Adobe. But the free software viewers use Poppler to render and display pdf files.
I have just found this bug report (which was fixed after half a year) about thread safety. That means: From January 2013 on the viewers should be able to render multiple pages at a time (if implemented in the viewer, poppler now does support this). Rendering of all pages one a dual core machine only takes 58% of the time a single core machine would need.
The second bug report is about multi-threaded rendering of each page. This second bug is still open and not accepted or worked on. If this second “bug” would be fixed, then rendering would become even faster. Especially for complex documents with few pages or if you don’t want to wait long for the first page to be displayed.
For very complex single paged documents I tried to use Inkscape. But as of this bug report from 2008, Inkscape is now (since 2012) using multiple threads for rendering filters. But not yet for the rest of the application, e.g. zooming in and out. Inkscape uses — like Poppler does — Cairo for rendering. That means until Cairo isn’t multi-threading capable, the per-page-rendering wont speed up using multiple threads separated over many cores.
In January 2013 someone started working on this (cairo multi-threaded image rendering).
About Adobe: Lets see who is faster in releasing the multi-threaded viewers.