you can make a formula field...
mymediaastext, text, = mymediafield
then mymediaastext contains a textual description of your media. that means file name (and pixel-size of pictures).
you can search for the file name of the media or document.
but you can *not* search for textual content of .pdf or .doc documents.
greetings from germany
Indeed kapitaen is correct that one can not search within a document natively. However, many use a plugin to extract the data into a FileMaker field and then search that field for it.