Smallpdf's font guide connects empty boxes in PDF text with missing or incorrectly embedded fonts and unsupported glyphs. A viewer may be unable to display the referenced characters even though the source document looked correct. Replacing the font and exporting again can address the underlying issue.

Illustrative image
Illustrative image.

Keep a copy of the original while checking the repair. Inspect the affected words across the target devices or readers, rather than assuming one corrected screen guarantees every character now renders properly.