I have just the steps you'll need to get that invoice properly printed. When you choose to Print a form in QuickBooks Online (QBO), the data is sent to your browser's native printing module to be completed. For certain browsers (such as Google Chrome), if a default printer hasn't been selected, it will attempt to download a PDF of the printer-friendly form instead. As you noted, if you don't have a PDF reader installed on your PC, this file will not let you open it.
The instructions needed to select the correct printing device will vary based on which browser you're using for QuickBooks. This can be accomplished in Google Chrome by following the steps below:
Selecting a printer in Chrome
From QuickBooks Online, navigate to the Sales tab > Customers section > click the customer's name > click their estimate/invoice from the list below.
Click Print or Preview. Verify the information in the preview, and click Print.
Chrome's own Print screen will appear. The printer is selected in the top-left next to Destination, click Change to choose the correct device.
The browser will remember your selection for future printing, but you can also change your default settings in Windows by clicking Manage next to Print Destinations > right-click your printer > Set as Default.
Doing this will ensure your estimate/invoice is properly printed every time. Please let me know if there's anything else I can do for you, the Community will always have your back. Thanks for reaching out, wishing you a safe and productive week ahead.