The search request was unable to connect to the Search Service.

This is an error I faced trying to set up a fast search centre in an OOTB environment.

TechNet provides four common causes for this issue.

1: The communication channels are not cleaned up correctly
2: The FAST Search Server 2010 for SharePoint query processing services are not running
3: The SharePoint certificate is not present on one or more of the FAST Search Server 2010 for SharePoint query processing servers
4: SharePoint has not been configured to point to the FAST Search Server 2010 for SharePoint query processing node

Reference: Technet

Search PDF files via Fast Search for SharePoint 2010

Fast Search for SharePoint 2010 searches PDF files by default(without any 3rd party plug-ins) given that they are not read protected or locked. When creating search scopes for Fast Search, the Document Library which contain the PDF files need to be in the included section of the search scope.

Mentioned below are some of the key points that come up when searching for the PDF search scenarios in Fast Search for SharePoint 2010.

• List of file types supported by default in OOTB sites – SharePoint 2010 does include PDF by default as search file types
• http://technet.microsoft.com/nb-no/library/gg471168.aspx
• Enable advanced filter pack – This is a solution to view content of PDF files in Search of SharePoint 2010
• http://technet.microsoft.com/nb-no/library/ff383314.aspx
• Configure iFilter to enable viewing of PDF content in SharePoint search – This is the recommended method to view content of PDF files
• http://support.microsoft.com/kb/2293357
• Adding searchable file types in SharePoint 2010 – Additional related info regarding adding searchable file types
• http://support.microsoft.com/kb/2518465
• Additional iFilter products
• http://blogs.msdn.com/b/opal/archive/2009/03/10/pdf-ifilter-battle-second-round.aspx
•  Select what contents are to be displayed in search results – This can be used to limit access to certain libraries, but it will have to be manually specified with this method
• http://office.microsoft.com/en-us/sharepoint-server-help/enable-content-to-be-searchable-HA010379092.aspx