How to combine pdfs generated by fpdi in different php files into one zip file

Question

I have a few php files that generate pdf files using fpdi. How can I combine all of the generated pdf files into one zip file for download in another php file?

Edit (to further clarify, also this is for a WordPress plugin I'm working on for my employer):

Here's an example php file (I'll call it 'generate_pdf.php') with the fpdi code that I'm using to generate one of the pdfs:

<?php
require_once( __DIR__ . '/inc/fpdf/fpdf.php' );
require_once( __DIR__ . '/inc/fpdi/src/autoload.php' );
require_once( __DIR__ . '/inc/fpdi-addOn/src/autoload.php' );

use \setasign\Fpdi\Fpdi;

// initiate FPDI
$pdf = new Fpdi();
$single  = $_GET['single'];

$pdf->AddPage( 'L' );

$pdf->setSourceFile( __DIR__ . "/inc/example_templates/example_template.pdf" );
$tplIdx = $pdf->importPage( 1 );

$pdf->useImportedPage( $tplIdx, 0, 0 );

$pdf->SetFont( 'Helvetica' );
$pdf->SetFontSize( 9 );
$pdf->SetTextColor( 0, 0, 0 );
$pdf->SetAutoPageBreak( false );

// stuff to write to the pdf

if ( $single === 'true' ) {
    $pdf->Output( 'D', 'example.pdf' );
} else {
    $pdf->Output( 'F', __DIR__ . '/pdfs/example.pdf' );
}

And here's the relevant part of an example generate pdf bulk action code:

echo "<div><a href='https://example.com/wp-content/plugins/plugin_name/generate_pdf.php?post={$id}&single=true'>Generate PDF</a></div>";

As you can see, in order to generate a single pdf for download, I have to click the link. This is fine for generating pdfs individually, but I have to write a bulk action script that generates multiple pdfs at once using several different php files with fpdi code, zip them, and download them, which I don't know how to.