I am using QGIS 3.4.
I'm creating a pretty simple map that shows major roads, cities/towns, and state/county boundaries for 4 northeastern U.S. states. All but the boundaries have labels (not buffered). I was successfully exporting large PDFs (22x36, 300 dpi) of this map as I progressed to make sure my file wasn't too large. At a certain point, my map stopped exporting, so I started turning layers off to see what would and wouldn't export. Now I'm at a point where I'm lucky if I can export just the basemap at 75 dpi. My maps renders at the same speed, but I get the 'bad allocation' notice within seconds of attempting to export anything.