Best internal documentation tool?

Try ProProfs Knowledge Base software. It can be the perfect fit for your requirements. The tool helps in creating internal documentation that can be accessed across multiple devices and browsers. It is packed with stellar features like rich text editor, branding and customization options, easy publishing, multiple language support, roles and permissions, and many more.

Check out Tettra. You can organize everything in Slack and have it available for your team without leaving chat.

The best collaborative documentation and works also offline. This is used internally by Product Hunt people!