How to protect pdf file with Tomcat filters?

I am currently running a tomcat instance with struts 1 and I would like tomcat to detect when pdf files are requested in the URL (For example of a link: http://www.***.com/files/action=download&name=myreport.pdf).

At this point I want a java class to be instantiated, then using a pdf API I want to inject a password to a file. The main point here is that I do not want to have the password store in the original pdf file I am serving instead I want the password to be injected at runtime by Tomcat.

Please let me know if you have any ideas, I did a little of research and I came across tomcat filters but I am unsure if this will resolve this problem.

Please note the passwords are store in a database table.

Thanks

From the filter we invoke a Java class to do the actual "injecting of password".

The entry in the web.xml will redirect your call to a particular filter.

<!--web.xml call all calls to .pdf will invoke the particular filter.-->
<filter>
   <filter-name>PDF Filter</filter-name>
   <filter-class>PDFFilter</filter-class>
</filter>
<filter-mapping>
   <filter-name>PDF Filter</filter-name>
   <url-pattern>*.pdf</url-pattern>
</filter-mapping>

//This is the actual filter
public class PDFFilter implements Filter 
{
    public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain chain) throws IOException, ServletException 
    {
        PDFPasswordInjector pdfPassInject = new PDFPasswordInjector();
        //use HttpServletRequestWrapper to get the pdf location/pdf name
        pdfPassInject.injectPassword( "<pdf location>" );

        chain.doFilter(request, response);
    }
}

//Java class to inject the password
public class PDFPasswordInjector
{
    public boolean injectPassword( String sPDFName )
        {
                // retrieve password from DB
                // use API to inject password to PDF
        }
}