Want to look up invoices by PO numbers

Thanks for posting in the Community, @gracebaptistchur. 

It seems that the purchase order (PO) numbers you're referring are issued by your customers. If so, let's first add a custom field for the PO numbers to appear on your invoices. Let me guide you how.

1. Go to the Settings (Gear) icon at the upper right.
2. Select Account and Settings under Your Company. 
3. Go to Sales from the left menu,
4. Click Sales form content.
5. Under Custom fields, enter PO Number in the Name field.
6. Mark the Internal and Public boxes.
7. Choose Save.
8. Hit Done.

The screenshot below shows you the last six steps. 

Once done, create an invoice and manually put the PO Number in the custom field. Then, click Save as seen in the screenshot below. Repeat the process for other invoices. 

After that, you can search for your invoices by using the PO numbers. I'll guide you how. 

1. Go to the Search (magnifying glass) icon at the upper right. 
2. Click Advanced search. 
3. Choose Invoices from the transaction type drop-down. 
4. Select Custom Field:Field 1.
5. Manually enter the PO number in the Enter Custom Field box. 
6. Hit Search.

See the screenshots below to show you the steps.

You can always run and customize the Transaction Detail By Account report in the program. This lets you view all your invoices that are due on a specific period. Just go to Reports from the left menu and enter the report name in the Search box. Then, click Customize, set the Transaction Type to Invoice, and A/R Paid to Unpaid. For more details, check out this article: Generate An Invoice By Due Date Report. 

I'm here anytime you have other concerns. Have a great day, @gracebaptistchur.