If you are the one sending the files, use the ETRANSMIT command. This creates a ZIP package that automatically includes all dependencies, including font files, so the recipient never sees the substitution error. 4. Map the Missing Font Permanently
A font on Windows (e.g., Calibri) may not map perfectly to the same font on macOS or Linux. Similarly, older PDF versions may not support modern OpenType features. Download Font Substitution Will Occur Continue
Here is a blog post explaining why this happens and how to fix it for good. Treatise on "Download Font Substitution Will Occur Continue"