Another convenient workaround is to export to PDF, reimport the PDF file and export it again. For my files it reduced sizes from 2 MB down to 200 kb.
Regards Patrick
Edit: Here is an example of an 700kb export. If reimported and reexported, the file size is down to 28k. Quality seems the same to me. Special characters however, like ä or ö from German are not converted correctly though. If anybody has an idea on how to deal with this?
Export["C:\Users\Desktop\test.pdf", "abc äüö"];
reimport = Import["C:\Users\Desktop\test.pdf"];
Export["C:\Users\Desktop\test2.pdf", reimport];