You can use a button in the Help Center window display pane (right side) to change the help text display size: click Increase Font Size or Decrease Font Size (repeatedly if necessary) until the size is right for you. (If the text appears blurry, try decreasing the size.)
Suggestion: Don't close the Help Center window, because font size changes will be in effect only for as long as the current window is open. Consider using the Keep on Top toggle in the Help Center window.