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.
Wordperfect files: libwpd.
RTF: unrtf
TeX: Recoll uses the untex program. Your distribution may have a package for it. If it doesn't, there is a copy of the source on the Recoll web site, because the program has no obvious home. The filter can also work with detex and will use it if it is installed.
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.
Pictures: Recoll uses the Exiftool Perl package to extract tag information. Most image file formats are supported.
Text, HTML, mail folders Openoffice and Scribus files are processed internally. Lyx is used to index Lyx files. Many filters need sed and awk.