Following the instructions below should change the font colour immediately as well as changing the default setting, so next time you open a PDF document it should open with your chosen font colour.
- In the current version of Adobe Acrobat Reader click on Menu, then Preferences. Alternatively press the shortcut key CTRL + K to open Preferences
OR in older versions of Adobe Acrobat Reader, click on the Edit menu, then Preferences.
- Under Categories, click on Accessibility
- Put a tick next to Replace Document Colours
- Choose Custom Colour
- Choose Document Text colour
- Click OK
Unfortunately, this may not work with all PDF files as it depends on how the PDF was originally created. You may be able to use SensusAccess to convert the PDF into a tagged PDF which may allow the font colour to be changed.