Do not save generic "PDF" files. Use print standards:
If you are the creator of the PDF, you made a mistake during export. You need to go back to your source file (InDesign, Illustrator, Word, Canva). Download Font Substitution Will Occur Continue
In Adobe InDesign/Illustrator:
Go to File > Adobe PDF Presets > Press Quality. In the export dialog box, click on the Advanced tab. Under "Fonts," ensure the box "Subset fonts when percent of characters used is less than..." is checked. This forces embedding. If the box is greyed out, the font distributor has blocked embedding. Do not save generic "PDF" files
In Microsoft Word (Windows/Mac):
Go to File > Options > Save. At the bottom, check "Embed fonts in the file" and check "Do not embed common system fonts." Re-save the Word document, then re-export to PDF. In Adobe InDesign/Illustrator: Go to File > Adobe
This issue is most common in PCL (Printer Command Language) drivers. PCL drivers rely heavily on printer-resident fonts for speed. If the driver is set to "Print Text as Graphics" or "Substitute Device Fonts," it may trigger this warning to ensure the user knows the output might differ slightly from the screen view.