linux: search grep all man pages

If you would like to perform some operation on linux, and is not aware of utility that need to be run, you can search all available commands man pages to search.

There is a fine utility to do so, called apropos.

For example, I want to know all commands that operate on pdf files, like conversion from pdf to something else and vice versa, I will run
uid@host:~/apropos pdf
acroread (1)         - Adobe PDF Viewer for viewing PDF files on X Window System.
dvipdf (1)           - Convert TeX DVI file to PDF using ghostscript and dvips
evince-thumbnailer (1) - create png thumbnails from PostScript and PDF documents
fix-qdf (1)          - repair PDF files in QDF form after editing
foomatic-ppdfile (1) - Generate a PPD file for a given printer/driver combo
ghostscript (1)      - Ghostscript (PostScript and PDF language interpreter and previewer)
gs (1)               - Ghostscript (PostScript and PDF language interpreter and previewer)
gsnd (1)             - Run ghostscript (PostScript and PDF engine) without display
pdf2dsc (1)          - generate a PostScript page list of a PDF document
pdf2ps (1)           - Ghostscript PDF to PostScript translator
pdffonts (1)         - Portable Document Format (PDF) font analyzer (version 3.00)
pdfimages (1)        - Portable Document Format (PDF) image extractor (version 3.00)
pdfinfo (1)          - Portable Document Format (PDF) document information extractor (version 3.00)
pdfopt (1)           - Ghostscript PDF Optimizer
pdfseparate (1)      - Portable Document Format (PDF) page extractor
pdftocairo (1)       - Portable Document Format (PDF) to PNG/JPEG/PDF/PS/EPS/SVG using cairo
pdftohtml (1)        - program to convert PDF files into HTML, XML and PNG images
pdftoppm (1)         - Portable Document Format (PDF) to Portable Pixmap (PPM) converter (version 3.00)
pdftops (1)          - Portable Document Format (PDF) to PostScript converter (version 3.00)
pdftotext (1)        - Portable Document Format (PDF) to text converter (version 3.00)
pdfunite (1)         - Portable Document Format (PDF) page merger
ps2ascii (1)         - Ghostscript translator from PostScript or PDF to ASCII
ps2pdf (1)           - Convert PostScript to PDF using ghostscript
ps2pdf12 (1)         - Convert PostScript to PDF 1.2 (Acrobat 3-and-later compatible) using ghostscript
ps2pdf13 (1)         - Convert PostScript to PDF 1.3 (Acrobat 4-and-later compatible) using ghostscript
ps2pdf14 (1)         - Convert PostScript to PDF 1.4 (Acrobat 5-and-later compatible) using ghostscript
ps2pdfwr (1)         - Convert PostScript to PDF without specifying CompatibilityLevel, using ghostscript
qpdf (1)             - PDF transformation software

run "man apropos" for extra features with it.