It’s not necessary to add clients into your database if you only want to let them download the report or invoices.
Let’s take “Invoices” as an example, when creating the “Invoice” sheet, please create a field with unique value to recognize which client the invoice belongs to. For example, their membership number, phone number, email address, or VAT number.
Once the sheet created, you can embed the sheet with database query & view and set the “Exact Mode”. Then, set the unique field as the query field.
In the future, when clients access your website, they can query the unique value (their phone numbers, email address, or something unique) and the system will return invoices that linked to the value they input. Then, they can print the invoice into PDF via the printing feature in their browsers. (Or, you can create a file upload field to store the PDF invoice for them to download as well.)
Note: if you think above will not work for your case, let me know and I can tell you the other method that add clients as the external users.
For the pricing, it depends on how many sheets and internal users you need. If you need more than 5 customized sheets and you’re the only user that needs to access the database, the monthly pricing would be $19. If you have 3 users that need to access the database, the monthly pricing would be $19*3=$54.