I don't think it's WebAssist's fault, this one. The Acrobat Reader version is whatever version the user has had installed by the browser he is using, - if he hasn't installed updated versions as they are issued (for whichever browser he's using) then there is a chance that some recently issued pdf's won't load properly. If you don't have Acrobat Reader installed as a browser add-on, it should download normally to your PC, and not try to open in a browser window.
The combination of browser types (times) browser version (times) Acrobat Reader version produces a huge number of possible scenarios for incompatibilities to cause errors, unless one is totally on-the-ball with getting the latest updates.
Right-clicking and 'saving as' is a good remedy at the moment (though forcing download would be easier - I think).