You can use pandoc at the command line but you'll need to go through pandoc documentation.
You'll have to create the markdown input for pandoc or generate it from Rmarkdown, then pass any argument you need though command line.
there is a keep_md argument in rmarkdown::*_document function to keep the md document generated, and there is a clean argument to set to FALSE to keep the intermediary files.
Rmarkdown is not really designed to be used at 50% of its process. It happends that some argument to pandoc command are temp file created by rmarkdown. You'll however find some pandoc functions helps like pandoc_convert - maybe they can help.
basically, the issue you have is that the input file for pandoc (abc.utf8.md) does not exists - you'll need to keep it, or create a new one to pass to pandoc. Pandoc take md inputs not rmd inputs.
Hope it helps.