Fix files getting cleaned.

src/Form/FillPdfOverviewForm.php

@@ -215,6 +215,7 @@ class FillPdfOverviewForm extends FillPdfAdminFormBase {
     // based off that.
     /** @var \Drupal\file\FileInterface $file */
     $file = $form_state->getValue('upload_pdf');
+    $file->setPermanent();
     $file->save(); // Save the file so we can get an fid
 
     $fillpdf_form = FillPdfForm::create(array(
@@ -246,4 +247,4 @@ class FillPdfOverviewForm extends FillPdfAdminFormBase {
       drupal_set_message($this->t("No fields detected in PDF. Are you sure it contains editable fields?"), 'warning');
     }
   }
-}
\ No newline at end of file
+}