When a PDF looks wrong — garbled text, missing images, strange colors, or content that just won't load — the cause is almost always one of three things: the viewer, the fonts, or the file itself. Narrowing it down takes about a minute.
Test With a Different Viewer First
Before assuming the file is broken, open it in a different PDF viewer. Chrome's built-in viewer is convenient but it's one of the weaker renderers — it handles basic documents fine but stumbles on transparency effects, certain blend modes, or PDFs using newer spec features. If the file looks correct in Adobe Reader but not in Chrome, the file is fine. Adobe Reader is free and worth having installed for exactly this reason.
Font Problems Are the Most Common Cause
If the viewer isn't the issue, fonts usually are. A PDF can either embed its fonts inside the file or just reference them by name. When fonts are only referenced, the viewer substitutes whatever it has available — sometimes that looks fine, sometimes you get boxes, question marks, or completely wrong characters.
CJK fonts (Chinese, Japanese, Korean) are particularly prone to this. Documents created on a machine with those fonts installed look perfect to the creator, but arrive elsewhere as unreadable symbols because the fonts weren't embedded. The same happens with certain decorative or specialty typefaces. In Adobe Reader, check File → Properties → Fonts — any font listed without "Embedded" is a potential problem.
If you created the document, re-export with font embedding enabled. In most applications this is a checkbox in the PDF Editor export settings. If you received the document from someone else, the simplest fix is asking them to send a new version with fonts embedded.
Transparency and Rendering Artifacts
White boxes over content, unexpected color shifts, or graphics that look completely different from intended — these are usually transparency rendering issues. PDFs exported from Illustrator or InDesign often use live transparency effects that some viewers can't process correctly. The creator can resolve this by flattening the transparency before export, which converts the effects into a static rendered layer that displays consistently in any viewer.
Incomplete Downloads
A PDF that downloaded with a broken connection will often open but show blank pages after a certain point, or crash the viewer mid-document. Check the file size — if it's noticeably smaller than expected, the download was cut short. Re-downloading on a stable connection usually solves it immediately.
One practical check before spending time troubleshooting: try printing the document. Display problems that look severe on screen often produce correct output in print, because the print driver handles rendering differently than the on-screen viewer. If the printed page looks right, the file itself is intact.