I am pretty new with KNIME and still struggling to find the "string to document" node. There are all kind of options such as "string to number" or "string to date" but nothing similar to document. Even in the help search menu I can't find "string to document".
You can find the String to Document node in the node repository under KNIME Labs -> Text Processing -> Transformation.
In general, if you can't find the node you are looking for in the node tree, simply enter its name in the Node Repository's search field. If you are unsure about the node's name, you can click the box to the left of the search field to enable fuzzy search.
The node you are looking for is called "Strings to Document" and it's part of the KNIME Labs Text Processing extension. You won't find it unless you have installed this extension.
To install the Text Processing extension, go to File --> Install KNIME Extensions... and look under KNIME Labs for Text Processing (or type it in the search box). Select the extension and follow the on-screen instructions to install it.