Recoll uses external applications to index some file types. You need to install them for the file types that you wish to have indexed (these are run-time dependencies. None is needed for building Recoll):
Openoffice: supported natively, but needs the unzip command to be installed.
PDF: pdftotext is part of the Xpdf package.
Postscript: pstotext.
MS Word: antiword.
MS Excel and PowerPoint: catdoc.
RTF: unrtf
dvi: dvips
djvu: DjVuLibre
MP3: Recoll will use the id3info command from the id3lib package to extract tag information. Without it, only the file names will be indexed.
Text, HTML, mail folders and Openoffice files are processed internally.