it seems that you are calling the API correctly and that a PDF is created, but you are converting http://pdfcrowd.com/api/pdf/convert/uri/ to PDF.
i.e. you set something like this
$fields['src'] = 'http://pdfcrowd.com/api/pdf/convert/uri/'
curl_setopt($c, CURLOPT_POSTFIELDS, $fields);
Try to change the src field to a page you want to convert, e.g.
$fields['src'] = 'http://bbc.co.uk'
Does this help?