fix pdf creation for very large exports (#1533)

src/Utils/MPdfConverter.php

@@ -34,7 +34,21 @@ class MPdfConverter implements HtmlToPdfConverter
     {
         $mpdf = new Mpdf(['tempDir' => $this->cacheDirectory]);
         $mpdf->creator = Constants::SOFTWARE;
-        $mpdf->WriteHTML($html);
+
+        // some OS do not follow the PHP default settings
+        if ((int) ini_get('pcre.backtrack_limit') < 1000000) {
+            @ini_set('pcre.backtrack_limit', 1000000);
+        }
+
+        // reduce the size of content parts that are passed to MPDF, to prevent
+        // https://mpdf.github.io/troubleshooting/known-issues.html#blank-pages-or-some-sections-missing
+        $parts = explode('<pagebreak>', $html);
+        for ($i = 0; $i < count($parts); $i++) {
+            $mpdf->WriteHTML($parts[$i]);
+            if ($i < count($parts) - 1) {
+                $mpdf->WriteHTML('<pagebreak>');
+            }
+        }
 
         return $mpdf->Output('', Destination::STRING_RETURN);
     }