There are many possible reasons for a poor quality PDF export. One of the most common is rasterization of the original vector data during the export process. This rasterization causes blocks, pixels, or grids to appear on the exported layer. This is because the plan contains a layer that cannot be exported as a vector (transparent layers, BMP image symbols...). As a result of rasterization, all layers below the data layer that contain BMP image symbols or transparency will be converted to a flat raster image in the export file. To avoid rasterization, BMP image symbols should be replaced by pure vector EMF images or by font-based point symbols for characters. Avoiding unnecessary use of layer transparency can also help avoid unwanted layer rasterization.
A Python script for identifying problematic layers can be found at the bottom of the following page: