How do I provide files for download?

I have written a post and would like to provide a file as attachment for download.

So far I have

created a link [Download files][download-file]
declared the link as [download-file]: download.zip
put download.zip in the _posts folder

However, the file does not get copied to the correct location.

I have

read about data files
read about directory structure
read about static files, which IMHO was the closest hit. Unfortunately, no example is provided.

So neither page gave me an answer on how to provide files for download.

I don't know where to put static files in the directory structure. Once I knew that, it seems I could use some Liquid like {{ site.static_files }}, but even then I'd still need to specify a file name.

Solution

Here is my solution. I have a "myname".github.io, I created a folder /download inside the myname.github.io and inside this download folder I uploaded the files I want to include in my posts.

In the single post then I created the following code:

some text and [here is possible to download the file in PDF][1]

[1]:{{ site.url }}/download/file.pdf