If you frequently face this warning, request that senders save files as PDF/A-1b (an ISO standard for archiving). PDF/A requires all fonts to be embedded and forbids substitution. If a file complies with PDF/A, the warning will never appear.
is not a minor glitch—it is a warning that your document is falling back on a guessing game. For personal use, it may be an occasional nuisance. For professional publishing, legal documents, or brand-sensitive materials, it is a red flag demanding immediate action. Download Font Substitution Will Occur