Hi Roberto,
You can speed up searches in large files by embedding an Index. In Acrobat X or XI, you can create the index using the Tools Pane->Document Processing->Manage Embedded Index.
In earlier versions ( 8 or 9) it is under Advanced- Document Processing->Manage Embedded Index. After you select that a message that your PDF doens't have an index will pop up. Go ahead and click Embed Index button and after it goes through creating and embedding the index the file will reopen, and hopefully your searches will be much faster.
Hope this helps,
Dimitri
WindJack Solutions
www.windjack.com
www.pdfscripting.com