Privacy and confidentiality.
Contents hidden in PDF or other files can be easily revealed. Similar is the position in other documents. So are the pictures.
I do feel concerned with this problem. As documents come to us from a variety of unconfirmed sources and we use them as per requirement and also upload and transfer these doc. (See http://www.metadatarisk.org/)
Metadata removal tools for Linux:
Pictures: Shotwell has an option not to transfer meta data. Similar option exist in DarkTable. Phatch also has some features.
Text: Text files are better than doc files which have more metadata, as to who contributed what.
for Windows, Linux (tarball and deb) and Mac:
Fedora etc. using yum: