I had the same issue and I received from Sitecore support next response (it works fine after):
1) Copy all the Adobe iFilter .dll files into the "\System32\Inetsrv" folder. This is the working directory for IIS on Windows Server. The Adobe iFilter .dll files are stored at the "C:\Program Files\Adobe\Adobe PDF iFilter 9 for 64-bit platforms\bin" folder by default. Also you can use the "IFilter Explorer" tool to detect the folder where the .dll files are stored: http://www.citeknet.com/Products/IFilters/IFilterExplorer/tabid/62/Default.aspx For more details please see the screenshot: http://screencast.com/t/xmWukanM+
2) Delete all the files under the "Website/App_Data/MediaCache" folder;
3) Rebuild the Sitecore Search Indexes (Sitecore -> Control Panel -> Indexing -> Indexing Manager);
4) Clear the Sitecore cache (the http://{hostname}/sitecore/admin/cache.aspx tool); 5) Restart the IIS;