PDF is being output to browser as strange character string

On occasions, mPDF can output the PDF to the browser as a string of weird characters, starting with %PDF-

%PDF-1.4 %���� 3 0 obj <> /Contents 4 0 R>> endobj 4 0 obj <> stream x��[�o�����j#���d���ٙ��)dYvlĎ�]��ئ-��^�(EpE�,Ăd!J

This is a binary PDF representation and it means Content-type: application/pdf header is not sent correctly or it is overriden by your code or setup. Most likely by text/plain or text/html.

Try to figure out these:

• Are you resetting Content-type header in PHP code somewhere after calling the mPDF Output method?
• Is your server forcing a different Content-type somewhere in your setup?
• Does your browser support displaying application/pdf Content-type directly?