Custom 404 redirect for a luddite

I’d suggest customizing your 404 template, within your theme. That way you can add a conditional that only sends people to the “not found PDF” if they were trying to access a PDF. If they were trying to access any other document type, like a webpage, that doesn’t exist, they will still see your regular 404 page.

To set this up:

If you are not already using a custom theme or a child theme, set one up. That way when you update your theme, you won’t lose your customizations.

Edit, or create, a file called “404.php” in your (child or custom) theme’s root directory. The contents should be something like this:

<?php
// check the requested URL
$requestUrl = "$_SERVER[HTTP_HOST]$_SERVER[REQUEST_URI]";
// see if the URL contains the "Images" folder
preg_match('/Images/', $requestUrl, $matches);
// if the URL matches, it will be a non-empty array
if(count($matches) > 0) {
    wp_redirect('your-pdf-url-here');
// if the URL didn't match, use the normal 404 template
} else {
    // your normal theme 404 code, starting with get_header();
    // ...
    // ...
    // don't forget the closing } at the end of the file
}
?>

There is one other alternative if you don’t want to edit theme files. You can create a plugin that does the same thing. Basically, you check if is_404(), then put in the same code – if the URL contains the Images folder, redirect to your pdf. You wouldn’t need an “else” condition, you’d just let WP do its normal thing.