impress odp file, export as pdf, reduce filesize

Impress, have thirteen files with 60-130 slides each, graphics and gradient backgrounds. Export as pdf with jpg compression 70 and 150 dpi yields some of reasonable size (.8 to 2.8 MB) and four that are: 4.8, 5.1, 9.8, and 12.4 MB.
Can someone help figure out how to reduce these four filesizes? I have tried everything I’ve found or been able to think of …
It appears to be due to the backgrounds, not my graphics.When I changed background to black, the filesize increased considerably.
Reducing the dpi or increasing the jpg compression recudes the filesize but the output resolution is too poor.
When I remove my graphics or reduce their sizes, the exported pdf filesize reduces only a little bit, not significantly.
Using several online pdf reducers does not help (I assume because I’ve already used jpg compression in LO)
If it would help, I can provide the links to the pdf files so you can see it. The biggest one (12.4 MB) is