pdftotext is a handy little tool to convert pdf files to plain text. It’s part of Fedora’s poppler-utils package and can be installed with yum:

# yum install poppler-utils

You can use pdftotext and the GNU findutils to search for a specific string in multiple pdf documents:

$ find /path -name '*.pdf' -ls -exec pdftotext {} - \; | grep -i "string"

See also pdftotext manpage, find manpage, grep manpage