My lowres pdf is 300mb, normally it should be about 10 mb max.

[hanneke61]

I want to create a PDF with the smallest file size. However, it will be 300 MB in size, while normally it is a maximum of 10 MB. I use large images, more than 600 dpi, but they should be automatically reduced in size with a low-res PDF. Is there a setting that is preventing it from working as I would expect? I already read something about downsampling, but I can't find that when I export.
Can anyone help me further?

[hanneke61]

finally it will be printed. But it concerns me that the file behaves so differently than I am used to. I'm afraid there is something wrong with the document that could also cause problems with printing.
I have now made JPFs from all illustrator files. This has returned it to its normal size and my problem is solved for now.

[leo.r]

Try to export your file as normal PDF (that is, Print, not Interactive). Set the standard to PDF/X-4 and resolution/compression to the desired levels. Will it make a difference?

EDIT: As I see, @rob day has just already mentioned the Interactive PDF aspect while I was writing.

[rob day]

Maybe export it on a certain way?
If I show a design to my client

Also, if the purpose of the PDF is for client review, try Exporting to a Print PDF with the Output tab set to this:

The above will flatten all live transparency and convert the color to sRGB, which will display properly even in browsers or PDF readers that can’t handle transparency effects correctly.

[hanneke61]

That is what I would also expect, but also this pdf is extreme large. 

[rob day]

Is the PDF/X-4 @leo.r suggested also large?

[hanneke61]

Yes, also that one is too large

[rob day]

Try creating a Transparency Flattener Preset that forces Rasters:

Then when you Export a flattened sRGB PDF use it in the Advanced tab:

[danpinho]

You placed complex AI/EPS (Transparency, Multiply, etc.)
To identify the problem, try converting them (the suspicious ones) to JPG (not ideal, I know) and save PDF again.

Hope it helps