while trying to inject an additional http header to mark the canonical url it is impossible to use the REQUEST_URI variable as follows:

    <IfModule mod_headers.c>
        <FilesMatch ".pdf$">
            Header add Link "https://EXAMPLE.COM%{REQUEST_URI}e; rel="canonical""
        </FilesMatch>
    </IfModule>

In the result the rule is applied and an additional header line is created but instead of the returned header lines content

https://EXAMPLE.COM(null); rel="canonical"

I would have expected to see

https://EXAMPLE.COM/PATH/TO/MY/FILE.pfd; rel="canonical"

Any ideas why this happens and how to overcome it? Apache version is 2.4.59

Kind regards
Nazdravi