AI answer, not guaranteed 100% accurate...
"EI does not automatically save PDF invoices to a fixed folder by default. Instead, when you want to save an invoice as a PDF, you must manually choose the location each time:
To save a PDF invoice, go to View > Invoices, select the invoice, click Save, and then select your desired save location on your computer.
The software allows you to choose where to store each PDF individually during the save process, rather than using a preset directory.
If you are looking for where Express Invoice stores its internal data (not the exported PDFs), on Windows systems, the data is typically stored in:
C:\ProgramData\NCH Software\ExpressInvoice (for newer versions of Windows)
C:\Documents and Settings\All Users\Application Data\NCH Software\Express Invoice (for Windows 2000/XP):
PDF invoices are saved wherever you specify each time you export or save them. There is no default PDF invoice folder unless you consistently select the same directory when saving"