PDF export is larger on lower presets than higher?

[Kimberli Burns]

Been around for several years so exporting PDFs is not a new concept but I had something happen I have not seen before. I receive files from a particular client that are intended for different usages, offset printing, trade shows, etc. and with that mind the placed images are sometimes quite high in DPI.

Tonight I exported a couple of these files using the "smallest file size" setting and the PDFs came out at 14.5MB. Looking at the file in Acrobat, the output settings in  indicated all placed images were 100dpi.

I exported the same file with PDFx1a and the file were sized at around 3MB.

What I noticed is the Compatibility settings were different. Smallest was Acrobat 6 and PDFx1a was Acrobat 4. I went back to Smallest, change the Compatibility to Acrobat 4 and sure enough it gave a super small PDF.

Is there a simple explanation as to why the Compatibility setting dramatically increases/decreases the file size or perhaps a resource someone could point me to? What I found on the Adobe site didn't really spell out differences in Compatibility settings.

Thank You.

Mac 10.12.6

Adobe CC - InDesign 13.1

[Cari Jansen]

My best guess is that there are significant structural differences between Acrobat 4 and Acrobat 6, e.g. layers are supported and number of other features that are not supported in Acrobat 4, which means MORE information is added under the hood of the PDF that is created.

[Test Screen Name]

When you save as PDF 1.3 all transparency is flattened. This is often not a good thing as it can make files bigger, but for some complicated designs they can get smaller.