PimiDocs is free, and updates are free with it — new releases appear on your Plugins screen like any other update, served from the w4dev.com plugin repository. No account and no license key are required for that.
The License page
You will still find a Docs → License page in the admin. It exists for the update integration and for anything premium PimiDocs may offer in the future; for the free plugin you can leave it empty. If you have been given a key — for example as an early-access user — enter it there and the page verifies it immediately, showing status, expiry and activation count.
Troubleshooting updates
If updates fail to appear, check that your site can reach w4dev.com over HTTPS — some server firewalls block outgoing requests. The plugin keeps working either way; only the update check is affected.