Currently, you cannot search the PDF content when it is placed on Cloud.
You need to use the Adobe Reader to search the PDF.
The Reader can be integrated with Acrobat.com and then you can open your files in Reader application and with ctrl-F you can search any word.
You can do some more search with multiple files using Advanced Search.
In the Reader application, choose Edit > Advanced Search.
Link to install Reader:
Adobe - Adobe Reader download - All versions
Regards,
Anoop