Name files so they sort themselves
Start every file name with the date in year-month-day order. Files then sort chronologically by default in every system.
Follow with the document type and the counterparty: "2026-04-12-invoice-northwind.pdf". Anyone can identify it without opening it.
Use hyphens rather than spaces, keep everything lowercase, and never include characters that break scripts and links.
Shallow folders, consistent structure
Deep hierarchies are hard to maintain and harder to search. Two or three levels is usually enough when file names are descriptive.
Organise the top level by year or by client, whichever you search by most often, and keep the second level identical across every branch.
Make everything searchable
Run recognition over scanned documents so their contents are searchable, not just their names. An archive you cannot search is a filing cabinet, not a system.
Store the searchable version alongside the original scan rather than replacing it.
Retention and backup
Know how long you are required to keep financial records in your jurisdiction, and do not delete before that. Set a review date rather than deciding case by case.
Keep at least two copies in different places, and test a restore occasionally. An untested backup is an assumption.