How to set resolution / DPI on convert-to pdf from ppt

I convert ppt and impress files to pdf using the options:

libreoffice --invisible --convert-to pdf --outdir some/dir inputfile.ppt

the resulting pdf has always just 72 DPI.
How can I set the resolution e.g. to 300 DPI?

Try making a first export from Menu/File/Exporta PDF and set up the resolution. I think it is saved for the next times.