Recoll is used to search for documents based on their content as well as thier filenames.
Recoll can search documents on many different formats and from a variety of storages (files, archive members, email attachments, etc while transparently handling decompression.
It is free on Linux, open source, and licensed under the GPL.
It is based on the very capable Xapian search engine library, for which it provides a powerful text extraction layer and a complete, yet easy to use, Qt graphical interface.