In general, to reduce the PDF file size you just have to decrease the DPI value, but it's not working with PDFCreator.
I open a web page with several pictures (e.g. Google Images), I press Ctrl + P, I select PDFCreator, and then I press Print. I select 100 DPI as resolution and then I click Save. The document title is
Now I do the same with 600 DPI (File:
Test_with_600_DPI). If I compare the size of the two files, it's more or less the same and I also can't see any differences in the quality. Why it is not working?