FontAwesome 5 mapping Error

Problem

I am using a custom document class from a book publishing company. When I include the fontawesome5 in it, the compilation gives me a mapping error. If I do not use the custom document class then everything compiles fine. For this reason, it may not make much sense to provide a MWE since it is a custom document class. I have an older version of fontawesome installed and it works fine.

Errors

/usr/local/texlive/2021/texmf-dist/tex/latex/fontawesome5/fontawesome5-mapping.def:22: LaTeX3 Error: Control sequence \faAdjust already defined.For immediate help type H <return>....                                              l.22 ...:nnnnn{\faAdjust}{adjust}{free0}{3}{"F042}

Questions

• I could not see it in the official documentation, but does fontawesome5 support PDFLaTeX?
• I have managed to get fontawesome5 to work with XeTeX and LuaTeX, but less successful with PDFLaTeX. How should I interpret the error message and is there a solution?
• I have also tried to install the fontawesome.otf font in my system (macOS Big Sur), but that did not seem to help. Is there something I could do to make latex recognize the font?

Official documentation

• FontAwesome 4: (works):https://ftpmirror1.infania.net/mirror/CTAN/fonts/fontawesome/doc/fontawesome.pdf
• FontAwesome 5 (does not work):https://ftp.acc.umu.se/mirror/CTAN/fonts/fontawesome5/doc/fontawesome5.pdf