PDF Image mode
If the PDF image mode is turned on, then all input elements are rasterized before converted to the PDF format (so the PDF file contains only this preprocessed raster image). This approach is useful if the converted plot file contains graphics entities, which are out of possibilities of the used PDF engine. So if the used PDF engine is unable to convert your plot file in standard mode, you can try to convert it in image mode.
The image mode has several options which can be set in the Advanced options dialog.