Actually this is known issue in version 5.0, where there is a support for resumable download. Regrettably in combination with Adobe PDFs (which has unexpected specification of bytes) it crashes. Fortunately within the hotfix 5.0.11 (well, you could rather apply the latest one), there is implemented workaround for this issue, so please try it.
It will be also necessary to add following key to your web.config file:
<add key="CMSGetFileAcceptRanges" value="false" />