Why this message?
PitStop checked the document for images that have been scaled non-proportionally.
Proportional scaling reduces the size of the image by an equal percentage along both the X-
and the Y-axis. If an image is scaled non-proportionally, the proportions between height and
length of the image are changed, resulting in distortions in the image.
Disproportional scaling may be allowed if the distortion is smaller than a specific value, because
very small distortions to the image may not be visually detectable.
Solutions
In your source application, replace the scaled image with the original image, and if required,
scale the image proportionally (i.e. equally along the X and Y axis). Re-create the PDF document.