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When printing your Mathcad files to pdfs it seems to save the gridlines as a picture behind the text. This can easily double the size of your files (see annex: pdf with excel-component (also a picture) created with adobe PDF). It is therefore wise to save pdfs with the gridlines deactivated.
Personally, I wish there was an option to view gridlines in Mathcad but not print them. i.e. view gridlines on screen only.