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It turned out, the problem was in the library i was using. I can't tell exactly what the problem was, but proc_close in the send method of the wkhtmlpdf class was returning 2 instead of the expected 0 when a PDF was succesfuly created. This lead to that the library thought that no PDF was created, and it simply returned false meaning nothing was outputted to the client. I solved it by checking if the file existed instead by using PHP's file_exists function.

php - wkhtmltopdf is returning blank pages for some urls - Stack Overf...

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