WEBSITE
On the left side of your document list select one or more check-boxes. Or click on the most upper check-box to select all items
The number of selected invoices and an Actions appears on the top of the page
Click on the Actions button
Choose between- Add payment (Invoices and Proforma invoices)- Export (CSV, XLS, PDF) - Delete
The chosen action will be performed, and a green message appears if successful
MOBILE APP
Tap on Actions
Select the documents, items or clients
Choose between- Add payment (Invoices and Pro forma invoices)- Export (CSV, XLS, PDF) - Delete
The chosen action will be performed, and an information message appears

You can read more about Adding payment here and here about the Export