It sounds like you're having issues with your mime_content_type configuration. I've copied the following from a previous post:
OJS uses PHP's mime_content_type function to determine file types; if that function doesn't exist, it calls the external UNIX standard "file" utility. Either way, it determines file types based on a file called magic.mime. I'd suggest making sure that PHP's mime_content_type function is properly configured. See http://www.php.net/manual/en/function.m ... t-type.php
The line <?php echo 'Mime type: ' . mime_content_type('pdf_file.pdf') . "<br/>\n"; ?> should return type application/pdf, but it's not, most likely due to a misconfiguation. If you'd like, point me to a pdf file on your server to look at, and I can double-check that it's not a document problem.
Alternatively, you can search the forums for mime_content_type -- this issue has been discussed before.