It is open source and free to use and can be easily integrated

The best aspect of Paperless for me was the built-in OCR (Optical Character Recognition) feature. In most apps, your scanned documents and photos would still be images – photos of text. Within Paperless you can mark certain parts of pages (such as emails, invoices or receipts) as covered-text and the software will try to directly extract that text, making it searchable and categorising the item appropriately. For example, if you decide that all of your incoming invoices should be indexed by a certain tax label, paperless learns that too and will start to apply automagic rules to new inflowing documents, doing your category selection for you. The more you use Paperless, the less manual work you have to do as it discovers your categorisation patterns.