Return an exception if passed file does not contain PDF headers

[voghDev]

The headers for PDF files can be seen in this link

Add a check that reads the first bytes and do not attempt to render the PDF if headers can't be found.

Must be cusotmizable, as new versions of PDF files may come with a different headers, and we need support for this. The check could also be disabled passing a parameter to the Builder, such as .doNotVerifyPdfHeaders()