What 1× and 2× actually mean
PDF page size is in points (1/72 inch). A US Letter page is 612×792 pt. At 1× we rasterize at 72 ppi: 612×792 pixels. At 2× we double that: 1224×1584 pixels, closer to a 150 ppi preview. A4 at 2× is about 1191×1684 pixels. This is not “whatever your monitor is.” A 27-inch display does not change the JPEG dimensions.
Quality 85% is a reasonable default (the slider). 50% is for chat. 95% is for a proof. A 12-page report at 2× / 85% is often 4–8 MB as a ZIP. Same report at 1× / 70% can be under 2 MB.
Text becomes pixels
After export you cannot select a paragraph. That is the point of a picture. If you still need text, keep the PDF or use Split. For Instagram’s 1080×1440 post, export at 2× then run Image Resize with the 3:4 preset.