TriumphS,
If you are using Sharepoint 2013, you must have the folder : %ProgramFiles%\CommonFiles\MicrosoftShared\webserverextensions\15\TEMPLATE\XML.
This folder is to checked on machine on which Sharepoint server is setup.
Also when you open the PDF in browser from sharepoint, does IE show any message in the ribbon saying that whether to enable "AcroPDF.dll" or "Adobe Acrobat Sharepoint OpenDocuments Component" .