pdf file not opening

I am able to open the file when testing the web site I created, but some of the users are having the following error:

There was an error opening this document. The file cannot be found.

I tested on my computer which has adobe acrobat pdf pro X , and all works fine, however the users are not being able to open the created pdf file

Any ideas what could be happening?

<?php
/*
Note that this line: /F (http://www.mysite.com/CompletionForm.pdf)
determines the PDF File to be filled in. 
*/

$txtName       = $_GET['txtName'];
$txtCourseName = $_GET['txtCourseName'];
$txtCompletionDate = $_GET['txtCompletionDate'];
//FOR TEST

//echo $txtName;
//echo $txtCourseName;


header("Pragma: public");
header("Expires: 0");
header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
header("Cache-Control: public"); 
header("Cache-Control: private",false); 
header("Content-Description: File Transfer");
header("Content-type: application/vnd.fdf");

echo <<<EOT
%FDF-1.2
1 0 obj
<<
/FDF
<< 
/Fields
[ 
<<
/T (txtName)
/V ({$txtName})
>> 
<<
/T (txtCourseName)
/V ({$txtCourseName})
>> 
<<
/T (txtCompletionDate)
/V ({$txtCompletionDate})
>> 
]
/F (http://appdevelsvr:8080/shquiz/media/certificate/SH_CertificateofCompletion.pdf)

>> 
>>
endobj
trailer
<<
/Root 1 0 R
>>
%%EOF
EOT;

?>

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