There is No way to change the darkness that prints from your terminal. If your terminal is printing too light you can try the following steps:

Make sure that you are using the power cord provided with your PAX A80. Failure to use the provided cord can result in power issues that cause your printer to print light

• Check the connection between the wall-outlet and the power cord
• Check the connection between the power cord and the power brick
• Check the connection between the power cord and the terminal
• Try a different paper roll, the roll of paper could have an issue

If this does not fix the issue you can try printing a different size to see if it makes it easier to read.