You designed this in a careful serif. The PDF is showing it in Times New Roman. Every elegant ligature gone, every kerning choice ignored.
The PDF didn't embed the font, and your viewer doesn't have it, so it's substituting a fallback.
What's actually going wrong
PDFs can either embed every font they use (self-contained) or reference fonts by name (relying on the reader's system). When fonts are referenced rather than embedded, opening on a machine that doesn't have those exact fonts triggers substitution — usually to a generic serif or sans-serif.
Licensed fonts often can't be embedded. Some export workflows skip embedding for size. And some PDFs reference fonts that don't exist anywhere, producing wild substitutions.
The quick fix
Re-export from source with 'Embed all fonts' turned on. In Word, this is in the PDF export options under 'Standards'. In InDesign, embed at export. In design apps generally, 'Subset fonts' or 'Embed all fonts' is what you want.
If you only have the PDF, convert to Word — Flint converts the displayed text (whatever font it's currently using) into editable text. Set the right fonts in Word and convert back to PDF with embedding on.
If that didn't work
Some commercial fonts have embedding restrictions. The PDF will silently substitute regardless of your settings. Check the font's license — if it's 'no embedding', you'll need to either license a version that allows embedding, switch fonts, or convert the text to outlines in the source app before export (which makes the text uneditable but visually correct everywhere).
For PDFs that look right on your machine but wrong on others, the issue is your-only embedding. The fonts exist on your computer but weren't embedded into the file. Re-export with embedding on.
Prevent it next time
Always embed fonts at export. Use fonts you have licensed for embedding. For documents that need to look identical everywhere, convert text to outlines at final export — fonts no longer matter because they're now shapes.
FAQ
Why does my PDF look right on my computer but wrong on others?
Your computer has the original fonts; theirs don't. The PDF references fonts rather than embedding them. Re-export with 'Embed all fonts' on and the PDF looks identical everywhere.
Can I embed fonts after the fact?
Indirectly. Convert the PDF to Word in Flint, the text becomes editable, set the fonts you want, re-export with embedding on. There's no in-place 'embed fonts now' operation.
What does 'subset fonts' mean?
Only the characters actually used in the document get embedded — a kind of compression for fonts. Saves significant file size with no visual difference. Always preferable to full font embedding for size, and to no embedding for portability.
Why is my PDF showing little boxes instead of letters?
The font is referenced but the reader can't find it, and the substitution failed. Common with Asian or symbol fonts on Western systems. Re-export with embedding to fix.
Embed your fonts. If the source PDF didn't, run it through Flint and rebuild with proper embedding turned on.