This question already has an answer here:
Certain (but not all) images displayed in
ArrayPlot, and then copied to PDF, show what appear to be fuzzy mesh lines in white areas. An example is shown below.
I am aware of previous answers regarding viewing in Preview, but this happens when the copied image is viewed in Pages, TextEdit, or even Mathematica! It persists in these when the preview smoothing option is turned off, though the problem goes away in preview.
Also, the PDF file is very large, takes long time to copy. Is it representing all the elements of the array as graphic rectangles?
Here is a simple example:
ArrayPlot[1. - IdentityMatrix, ColorFunction -> GrayLevel, ColorFunctionScaling -> False, Mesh -> None]
Copy the output as PDF, then paste back into the Mma notebook, and
Mesh lines will be evident.