There are quite a few options that can be used: (both tested).
Here are two ways.
header("Content-type: application/pdf");
header("Content-Disposition: inline; filename=filename.pdf");
@readfile('path\to\filename.pdf');
or: (note the escaped double-quotes). The same need to be use when assigning a name to it.
<?php
echo "<iframe src=\"file.pdf\" width=\"100%\" style=\"height:100%\"></iframe>";
?>
I.e.: name="myiframe" id="myiframe"
would need to be changed to:
name=\"myiframe\" id=\"myiframe\"
inside PHP.
Be sure to have a look at: this answer on SO for more options on the subject.
Footnote: There are known issues when trying to view PDF files in Windows 8. Installing Adobe Acrobat Reader is a better method to view these types of documents if no browser plug-ins are installed.