How to make PDF text easier to read

The biggest factor: contrast between text and page

Readability depends heavily on the difference between the text and its background. Accessibility guidelines put numbers on it: a contrast ratio of at least 4.5 to 1 for normal text and 3 to 1 for large text, with 7 to 1 as the stricter level. For reference, black on white is 21 to 1, #767676 grey on white is about 4.5 to 1, and #999999 is about 2.9 to 1, which fails.

A scan can fail that test without anyone noticing. Faint ink on greyish paper can be well under the minimum, and it is exactly the case where raising contrast helps most.

Four things you can change in a finished PDF

• Text darkness. Pull grey text towards black. Darken PDF does this for text pages by rewriting the colour, and for scans by re-toning the image.
• Background. A grey or cream cast on a scan lowers contrast for everything on the page. Whiten PDF background lifts the paper to white and leaves dark ink alone.
• Edge crispness. Soft edges make small text harder to resolve. Sharpen PDF steepens edges; it helps slightly soft scans and cannot rescue badly blurred ones.
• Stroke weight. Thin type benefits from a slightly heavier stroke, especially on paper. The Thicken thin text option does this on text pages.

Screen versus paper

A screen is backlit, which flatters weak contrast; paper only reflects the light in the room. A page that is acceptable on screen can be hard to read when printed. If the destination is paper, judge the result on paper, or use the PDF print optimizer, which measures the file and recommends settings.

Pure black on pure white is the maximum contrast, but some readers find very bright white tiring in a dark room. That is a viewing choice rather than something to bake into the file; dark modes in viewers exist for it.

A short readability checklist

1. Zoom to 400% on small text. Is it dark and solid, or pale and soft?
2. Is the paper white, or is there a cast or a shadow?
3. Is the smallest text thick enough to hold together? Test one printed page.
4. For scans: was it captured at 300 DPI? Below about 200, small print cannot be resolved however it is processed.
5. Compare before and after before you commit. Over-correcting turns light details such as pencil and stamps into blobs or blanks.

When processing will not be enough

No tool adds detail that was not captured. A very low-resolution scan, heavy motion blur or text that has faded to the colour of the paper cannot be made readable by tone changes. If the original paper exists, rescanning at 300 DPI in greyscale is the better fix.