I am selling PDF Search Engine with these features :
- Related terms on search result. This will boost clicks on result page
- Paging on search result
- Bad words blocking
- Search engine friendly URL.
- Search result cache on flat file on your server.
- Tracking of most clicked URL on each search result.
Live demo :
http://www.xrvel.com/sub/pdf-search/
Server requirements :
- Ioncube Loader
- MySQL (database is needed only to record hits, NOT search result)
- Apache mod_rewrite
- cURL
- Latest PHP
The core script is encoded by Ioncube. But the design files are not encoded.
A backlink to my web (xrvel.com) will be attached to the script automatically.
More information can be read on DigitalPoint : 
http://forums.digitalpoint.com/showthread.php?t=2133748