This is not really an anwer, but rather a workaround:
The LO-developers set up some sensible default for PDF-export, I don’t know of a way to fiddle with those except in the Presenation Minimizer in Impress. But if you know what you want, you can use Ghostscript to toast your PDFs to your liking:
Install ghostscript, then open Terminal in the working folder, and issue:
gs -dPDFSETTINGS=/prepress -dSAFER -dCompatibilityLevel=1.5 -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -sstdout=%stderr -dGrayImageResolution=600 -dMonoImageResolution=1200 -dColorImageResolution=300 -sOutputFile=outputfile_gs.pdf -c .setpdfwrite -f inputfile.pdf
This is just an example, which for example transformed an 8.4Mb PDF-presentation (with loads of PNGs) to a totally usable 2.4Mb PDF-file, w.o. visible loss of quality. Better experiment with the settings though.