OCR works best on scans at 300 DPI or higher. If your source is 150 DPI or below, accuracy drops to 80–90%; at 100 DPI it can fall below 70%.
If you can't re-scan, try running Compress first with the High Quality preset. It does a light denoise pass that often improves OCR confidence by 5–10 points.
For non-Latin scripts (Devanagari, Arabic, CJK), make sure you have explicitly chosen the language in the OCR settings — auto-detect is conservative and will fall back to English when in doubt.