Hello there, Karen.
Let's run some steps to fix why your device freezes when modifying invoices.
I recommend resetting the app data. Resetting the app data plays the same role as clearing the cache from your web browser. Here's how:
1. Click the Help menu at the top of the screen.
2. Select Reset App Data on the drop-down selection.
For additional troubleshooting options for Andriod, you can check out this article: Fix common errors in the QuickBooks Online mobile app for Android.
As for a handy hardware that allows you to print and sign invoices, this option isn't available in QuickBooks Online application. However, you can do so using a web browser. You can check this reference for the detailed steps: Print Multiple Invoices.
Keep us posted if you have other concerns with invoices.
