I'm not sure it's Archicad's fault.
Yesterday I published a 16-page PDF from Archicad (Merge pages on).
1 floorplan (lines and solid fills*), 2 sections (lines and hatched fills), 4 elevations (internal engine color shaded renderings/default settings), 5 pages of perspectives (live internal engine 3D color shaded views), and 3 pages of text (four fonts). Large part of the model is an average-complexity terrain mesh, with contours and lots of variously shaded polygons.
In average, the pages were about 80 kB each.1.2 MB in total.
I tried "Shrink filesize" in Adobe Acrobat. It grew slightly bigger! (That means Acrobat thinks Archicad's pdf is a small as it gets).
As email, the file grew to 1.6 MB (this is due to the normal text encoding).
I would guess it's your content that makes it big. Avoid bit-maps, jpegs and complex hatches. Use interna engine shading when possible, insead of open gl or rendered jpgs.
*Of course I modeled. I'm describing the content as it looks in the PDF!
AC4.1-AC26SWE; MacOS13.5.1; MP5,1+MBP16,1