From the main screen of any invoice or estimate, you can find the Print option by tapping the 3 dots icon located in the upper right corner.
After pressing on Print, you'll be presented with the Printer Options from your operating system. On this screen, you can select the printer you want to use as well as change any printing settings according to your preferences.
Please note that on Android, you need to connect your printer to Google Cloud Print, for more details, please check this article from Google: https://support.google.com/chrome/answer/1069693
You can also use the print feature to save a PDF copy of the invoice to your device, you can check how to do that in this article: