If the export frequency under 'Integrations', 'Default file settings' is set to 'Never', or if you want to export the documents for several client files manually in the meantime, proceed as follows:
Go to the 'Files' menu, select the client files for which you want to export the purchase and/or sales invoices, and then click the 'Export documents' button at the top.
In the pop-up that appears, you can specify up to and including which date you want to export the invoices. Then click 'Export'.
The invoices are now exported via all active integrations of the various client files.