Getting gibberish, sometimes, in PDF output

hi I'm generating a pdf file and giving users a link to the file (an online certificate). The link is just a regular link with a query string. Sometimes in the browser the pdf file opens correctly, and other times the page spews out gibberish (uncompiled). I am including the header header("Content-type: application/pdf"); in the file and it is on the first line (before includes, etc.). This is the first time I am outputting pdf files and suspect that probably I am missing something simple -- a step, call to a plugin, header, something. Can anyone tell me what that might be? thanks, s.