The person who created the PDF optimized the file size by choosing not to embed the fonts directly into the document. They assumed your system would have them.

If you know the author of the document, ask them to re-export the PDF with the "Embed All Fonts" option checked, or ask them to "print to PDF" as an image to avoid font dependency altogether. Summary Checklist

Open your terminal and run: sudo apt-get install poppler-data

Think of them as generic tags the system creates when it cannot identify a specific font family (like Arial or Times New Roman). For instance, when you convert a Word document to PDF, the software might internally label the embedded font as CIDFont+F1 as a processing step, which can sometimes lead to display errors.