Access row value in template_preprocess_views_view_fields(&$variables)
values is protected, so you have to use the get() method:

$variables['row']-&gt;_entity-&gt;get('field_file_pdf_upload')-&gt;target_id;


For the field object this shortcut is available (this is not standard OOP and works for fields only):

$variables['row']-&gt;_entity-&gt;field_file_pdf_upload-&gt;target_id;