PressLint production guide
How to check image DPI inside a PDF
Print sharpness depends on how many image pixels cover the physical area where the image is placed. Effective DPI changes when the PDF scales the image.
Find the raster image dimensions
Record the image resource's pixel width and height. Metadata DPI alone is not enough because the page can place the image at a different scale.
Measure the placed size
Read the page transformation used to draw the image and convert the physical width and height to inches.
Calculate effective DPI
Divide pixels by placed inches for each axis and use the lower result as the conservative effective resolution.
Compare with the print requirement
The acceptable threshold depends on viewing distance, process and printer. Treat a low-DPI warning as evidence for discussion, not an automatic statement of print failure.