Overview
Technical documentation, sometimes called the technical file, is the evidence a manufacturer puts together to show that a product follows the rules. It is not handed to customers by default. The manufacturer keeps it and produces it if an authority asks.
For a product under RoHS, the file is compiled to the standard EN IEC 63000, which sets out what RoHS evidence should look like.
What it usually holds
Bill of materials
What the product is made of, down to the parts
Supplier declarations
Statements from suppliers on their components
Test results
Analytical or lab results where they exist
Retention
Kept for about ten years in many cases
Where it fits
The documentation is the proof behind the paperwork on the outside. The EU Declaration of Conformity makes the claim, and the CE marking signals it on the product. The technical documentation is what you fall back on if anyone asks you to show your work.
Note: general educational information, not legal advice. Check the official source before relying on it.