How Nuget copies files to the output directory when indirectly referenced by a project?

I tried to build a nuget package to copy some files to the output directory of the referenced project, and I did so as follows:

1. Create [projectName].targets file.
2. Use the csproj file to package the files to be copied with the [projectName].targets file.
3. Call Copy in the [projectName].targets file.

This works when the nuget package is referenced directly, but not when it is referenced indirectly, how can I solve this problem?