Begining to code the Ajax action

assets/jquery.ajax.js

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+/*
+ * vim: set ai sw=4 smarttab expandtab: tabstop=8 softtabstop=0
+ *
+ */
+
+jQuery(document).ready( function($){
+  jQuery('.wpdfh_print_checkbox').on("change", function(){
+      $.ajax({
+        url: wpdfh_ajax_object.ajaxurl, // this is the object instantiated in wp_localize_script function
+        type: 'POST',
+        data:{ 
+            action: 'wpdfh_set_print_metadata', // this is the function in your functions.php that will be triggered
+            post_id: $(this).data('post_id'),
+            checked: $(this).prop('checked')
+        },
+        success: function( data ){
+            console.log( data );
+        }
+    });
+    })
+});
+

assets/wp-pdf-helper-admin.css

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+/*
+ * vim: set ai sw=4 smarttab expandtab: tabstop=8 softtabstop=0
+ *
+*/
+
+.column-wp-pdf-helper {
+    width: 50px;
+}

assets/wp-pdf-helper-print.css

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+/*
+ * vim: set ai sw=4 smarttab expandtab: tabstop=8 softtabstop=0
+ *
+*/
+
+video, 
+.attached-videos,
+header, 
+.plyr, 
+.leaflet-marker-photo, 
+.leaflet-top, 
+#wpgpxmaps_downloadfile,
+div:empty,
+nav,
+.wp-block-comments,
+.wp-block-query,
+footer,
+.other-posts,
+.no-print,
+#wpadminbar,
+.skip-link.screen-reader-text
+{
+	display: none !important;
+}
+
+html,
+main,
+#post-content {
+	padding: 0px !important;
+	margin: 0px !important;
+}
+
+.gallery,
+.attached-gpx {
+    page-break-before: auto; /* 'always,' 'avoid,' 'left,' 'inherit,' or 'right' */
+    page-break-after: auto; /* 'always,' 'avoid,' 'left,' 'inherit,' or 'right' */
+    page-break-inside: avoid; /* or 'auto' */
+}
+
+#post-grid {
+	display: flex;
+}
+
+#post-grid-column2 {
+	flex: 70%;
+	padding-left: 10px;
+}
+
+.attached-gpxes {
+	flex: 30%;
+}
+
+.wpgpxmaps_summary {
+	font-size: var(--wp--preset--font-size--small);
+}
+
+.leaflet-control-attribution {
+	font-size: 8px;
+}
+
+.date-prefix {
+	text-transform: capitalize;
+}
+
+.author-line, .author-line > * {
+	margin-bottom: 0px !important;
+	margin-top: 5px !important;
+	font-size: var(--wp--preset--font-size--medium) !important;
+}
+
+.entry-content, wp-block-post-excerpt {
+	text-align: justify;
+