To make small text more readable:

Choose Preview > Preferences, and click PDF.

Deselect “Anti-alias text and line art.”

Enter a smaller number in the “Greeking threshold” field.
When you view a PDF document in Preview, small text may be illegible or look fuzzy. You can change a few options to make the text more legible.
Choose Preview > Preferences, and click PDF.
Deselect “Anti-alias text and line art.”
Enter a smaller number in the “Greeking threshold” field.
Text in a point size smaller than the greeking threshold appears as a gray blur. The blurred text fits the outline of what the text would actually look like, but is illegible. Legible text may take longer to display because Preview must render each character individually.
If you turn off the “Anti-alias text and line art” option, small characters may appear fuzzy, and large characters and line art may appear jagged instead of smooth.