Convert color to gray, removes all color information from your PDF and make them Black and White, which reduces the filesize for PDFs that don’t need color.
Often scanned black and white letters still get saved with color information which then can be removed by this option.
Don’t unembed Fonts, normally we want to remove embedded fonts, as these often are present on all systems anyway. But sometimes some special fonts (especially these with umlauts) look strange when the fonts get unembeded. So this option will fix these font appearance problems but produce bigger files.
Don’t resize images, leaves images in their current state, which can fix some problems with special PDFs or with PDFs that use JPEG2000 and would get bigger by processing.
Create PDF backups, saves the original PDF in case you need it later for some reason.