Thanks for following up about sending purchase orders through QBO mobile app. 

At the moment, this feature is still unavailable. You'll want to consider using a third-party application that's compatible with QuickBooks to do so. 

You can visit our QuickBooks Apps website and search for one through this link: https://quickbooks.intuit.com/app/apps/home.

In the meantime, you may log in to your QuickBooks company through a browser from your phone or tablet to send your purchase order. 

For future reference, check out this article to learn more about adding and customizing fields to sales forms and purchase orders in QuickBooks Online. 

Let me know if you need more help about sending sales forms to your customer. I'll be here to guide you more.