For the digital forensics analyst, they are breadcrumbs to hidden payloads. For the document engineer, they are obstacles to clean rendering. For the everyday user, they are invisible ghosts.
These are arbitrary "tags" assigned by the PDF generator. In a single document, F1 might be Arial Regular , F2 might be Arial Bold , and F3 might be a specific symbols font.
: The original font wasn't properly embedded in the file. The PDF viewer knows the shape of the letters (glyphs) to show you, but the underlying "font file" is missing.
Errors typically occur when your PDF viewer cannot find the original font on your system and the PDF itself doesn't have the font "embedded" (stored inside the file). This often results in: CID+ Fonts - Adobe Community cidfont f1 f2 f3 f4 f5 f6
Search for the official (Continuous Release) on Adobe's website.
# Extract the font file if embedded if font["embed"]: font_data = doc.extract_font(font["name"]) with open(f"extracted_font['name'].otf", "wb") as f: f.write(font_data[0])
: Manually telling your software to replace the "ghost" F1 with a real font you actually own, like Arial or Helvetica. For the digital forensics analyst, they are breadcrumbs
If a PDF is compressed aggressively, downloaded over an unstable internet connection, or exported from outdated software (like old CAD programs or legacy ERP systems), the internal font map tables can break. The reader sees the code F1 or F2 but the link to the actual visual shape of the letter is broken. How to Check Which Fonts Are Broken
the original font into the PDF correctly. When you try to open that file on a different computer, your PDF reader looks for the original font, can't find it, and sees the generic "CIDFont+F1" label instead. Since it doesn't know what the font is actually supposed to look like, it fails to render the text. CIDFont+F1 issue - Adobe Community
Use a tool like pdffonts (part of Xpdf/poppler): These are arbitrary "tags" assigned by the PDF generator
If you own the professional version of Adobe Acrobat, you can permanently repair the font mappings using the Preflight tool. Open the PDF in Acrobat Pro. Search for the tool in the right-hand pane.
Upload the PDF to a premium PDF editor (like Adobe Acrobat Pro) or a free online OCR converter (like Smallpdf or PDF2Go). Run the tool.
, provide a tool to convert these CID-encoded characters into vector outlines.