8MB is nothing, I have successfully created up to 350 MB A0 sized PDFs from within Pro and ArcMap, both as 100% vector data, and with mixed vector / raster. In the latter case, I also used 300 dpi output setting, and the raster layer was the ESRI "Terrain: Multi-Directional Hillshade" from the Living Atlas. Quality setting Best.
I have never had the issue that you describe, needing to reduce the quality setting to "Better" to be able to export.
And yes, maybe surprisingly, Adobe Reader XI can open these monstrous PDFs, albeit with a mild waiting time before display. It only runs out of memory when I open multiple.
This in on a Core i7 HQ quad core laptop with 16GB RAM with dedicated video card.