FWIW. A few thoughts:
(1) If you are a print service provider providing a “proof” to a customer in PDF form, it really should be a raster PDF file coming directly from the same RIP used for printing. Each page should be a single ZIP-compressed (to avoid compression artifacts that accompany JPEG compression) CMYK (+ spot channels if appropriate) raster image at device resolution. Either each image needs to be tagged with a CMYK ICC profile or the file should be a PDF/X-4 file with an output intent matching the CMYK printing condition. Otherwise, what you have is anything but a real digital proof but more like a restatement of the PDF originally provided.
(2) Viewing such a PDF file requires a viewer that properly converts such raster images from the designated CMYK color space (per above) to the RGB color space of the monitor. Such conversions are not possible with MacOS or iOS Preview, Google's PDF viewer, or Microsoft's Edge browser's PDF viewer. Adobe Acrobat, Adobe Reader, and other real PDF products do support such proper viewing.
(3) And of course, if a customer is relying on soft proofing, their monitors should be calibrated.
If the above conditions are not in place, both the print service provider and the customers are simply fooling themselves.
- Dov