Can you possibly post a sample page of this PDF someplace for us to look at?
I have the same problem. Below, an image (A) of some texte from the PDF opened in Adobe Acrobat Pro 9.3. And an example (B) of how it should looks. You can see a stroke like if the text is outlined...
When I open the PDF in Photoshop, there is no font problem, only in Acrobat.
I have a 74 pages document I have to turn in Interactive PDF, and all the titles looks like crap between 50% and 200%.
Is there transpaency on the page?
Yes, I have a Multiply effect on images.
But guess what... today, the same PDF looks perfect...!?
A random bug?
edit: Had the same problem with another PDF, just closed and reopened Acrobat, and the PDF is OK.
Transparency tends to make text look bolder on screen in ID and Acrobat.
The solution is usually to make sure the text is on a higher layer, or higher in the stacking order than the transparency. When text interacts with transparency, part of it can be rasterized, causing it to thicken. (The appearance is similar to outlined type, although with outlined type the cause is because the type has lost its "hinting".)
Text on pages with transparency often looks bolder even if not being rasterized or outlined and when on its own top-level layer. This has been an ongoing complaint since transparency support was introduced.
If it is possible to set the text to overprint (without changing the design) it solves the preview problem.