I was facing the same problem. Here is a solution that finally worked for me.
%ProgramFiles%\Common Files\Microsoft Shared\web server extensions\14\TEMPLATE\XML\DocIcon.xml
There you should see a line
<Mapping Key="pdf" Value="icpdf.png" OpenControl="PdfFile.OpenDocuments"/>
Change it to
<Mapping Key="pdf" Value="icpdf.png" OpenControl="SharePoint.OpenDocuments"/>
(if you don't want the SharePoint open file prompt use this
<Mapping Key="pdf" Value="icpdf.png" OpenControl=""/>)
and do IISRESET.
But to note, there are some drawbacks
- You have to do this step on each front-end web server (or deploy a farm feature that does the change)
- Your change may be overwritten in future versions
If you modify DocIcon.xml, you must make identical changes on each front-end web server so that all servers remain in sync. A good way to do this would be to distribute changes to DocIcon.xml as part of a farm-level Feature. In addition, you should be aware that changes that you make to DocIcon.xml may be overwritten when you install updates or service packs to SharePoint Foundation, or when you upgrade an installation to the next product version.
See this link: Understanding DocIcon.xml Files