Search notes:

C:\users\username\Documents

The Documents special folder (aka my documents) is different from the MyDocuments virtual folder which appears under the Desktop virtual folder in the Windows Shell Namespace.

Determining the location of the Documents folder

In PowerShell, the location of the Documents folders can be queried with 
[environment]::getFolderPath('mydocuments')

Some subdirectories

Some subdirectories that might be found below the Documents folder include

R: tilde

R on Windows expands the tilde to C:\users\username\Documents which, IMHO, takes some getting used to: 
> path.expand('~')
[1] "C:/Users/Rene/Documents"

See also

The VBA function curDir() seems to return this directory path by default.
The .NET enumeration System.Environment+SpecialFolder.
The set-locationDocuments function of the PowerShell module filesystem.

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