XQuery script creates only 1 PDF file instead of many

Question

We are running an eXist database with several XQuery scripts. We also use software from a third party provider that runs server-side. This software takes XML files from the eXist db and creates PDF files from it which are then saved in the eXist db. This process is usually started via a button in Oxygen XML Editor so only one XML file can be converted to and saved as PDF at a time.

Now I wrote a script to automatically convert all XML files to PDF and save them in the eXist db. It is supposed to do the following: To "enter" each of the directories/collections listed in the function local:preview-all-collections. Then iterate over all XML files in each collection and convert each XML file to a separate PDF file. In the end, each PDF file is stored in the collection /db/data/Handschriften/Auto-Satz.

Unfortunately, the script creates only 1 PDF file and then seems to stop. Am I doing something wrong while iterating over all XML files? Do I type return too early? Or what is my mistake?

This is what the script looks like:

xquery version "3.0";

import module namespace file="http://exist-db.org/xquery/file";
import module namespace xmldb="http://exist-db.org/xquery/xmldb";
import module namespace response="http://exist-db.org/xquery/response";
import module namespace request="http://exist-db.org/xquery/request";
import module namespace functx="http://www.functx.com";
import module namespace process="http://exist-db.org/xquery/process" at "java:org.exist.xquery.modules.process.ProcessModule";

declare variable $preview-home := "/opt/tustep/preview_hs/";
declare variable $satz-dir-path := '/db/data/Handschriften/Auto-Satz'; (: $dir-path || '/Satz'; :)

declare function local:preview-all-collections() {
    let $collections :=
        <collections>
            {local:preview-collection("xmldb:exist:///db/data/Handschriften/G_Korrespondenz/Amtskorrespondenz")}
            {local:preview-collection("xmldb:exist:///db/data/Handschriften/G_Korrespondenz/Amtskorrespondenz/Kopien_Typos")}
            {local:preview-collection("xmldb:exist:///db/data/Handschriften/G_Korrespondenz/Drittbriefe")}
            {local:preview-collection("xmldb:exist:///db/data/Handschriften/G_Korrespondenz/Drittbriefe/Kopien_Typos")}
            {local:preview-collection("xmldb:exist:///db/data/Handschriften/G_Korrespondenz/G_Materialien")}
            {local:preview-collection("xmldb:exist:///db/data/Handschriften/G_Korrespondenz/G_Materialien/Weitere_Schriften_AB")}
            {local:preview-collection("xmldb:exist:///db/data/Handschriften/G_Korrespondenz/Privatkorrespondenz")}
            {local:preview-collection("xmldb:exist:///db/data/Handschriften/G_Korrespondenz/Privatkorrespondenz/Kopien_Typos")}
        </collections>
    return $collections
};

declare function local:preview-collection($collection as xs:string) as element()* {
    if (xmldb:collection-available($collection)) then (
        for $child in xmldb:get-child-resources($collection)
        let $file-path := concat($collection, '/', $child)
        where contains($child, ".xml")
        return <file>{local:run-preview($file-path)}</file>
    )
    else()
};

declare function local:run-preview($file-path as xs:string) as element()* {
    let $options := <options><workingDir>/opt/tustep/preview_hs</workingDir></options>
    let $xml-file-name := functx:substring-after-last($file-path, "/")
    let $file := util:base64-encode(serialize(doc($file-path)))
    let $pdf :=
        if (file:serialize-binary($file, $preview-home || "preview_hs.xml"))
        then
            if (process:execute(($preview-home || "preview_hs.sh"), $options))
            then file:read-binary($preview-home || "preview_hs.pdf")
            else ()
        else ()     
    return
        if (exists($pdf))
        then (
            response:stream-binary($pdf, 'application/pdf', local:pdf-file-name($xml-file-name)),
            xmldb:store($satz-dir-path, escape-uri(local:pdf-file-name($xml-file-name), true()), $pdf, 'application/pdf'),
            sm:chown(local:pdf-file-name($xml-file-name), "admin"),
            sm:chgrp(local:pdf-file-name($xml-file-name), "nutzer"),
            sm:chmod(local:pdf-file-name($xml-file-name), "rwxrwxrwx")
        )
        else
            <html>
                <head><title>HKG Preview</title></head>
                <body>
                    Der Previewprozess ist leider fehlgeschlagen!
                </body>
            </html>
};

declare function local:pdf-file-name($xml-file-name) {
    substring-before($xml-file-name, ".xml") || "_" || replace(substring-before(string(current-dateTime()),'.'),':','-') || ".pdf"
};

local:preview-all-collections()

My code might loke a bit non-elegant as my XQuery skills are pretty low and I am still struggling with file iterations. Would be thankful for any advice.

If you need more information please let me know. Thanks in advance.