Please follow the link , it might help you.
Use "file:///" before giving your absolute path.
Also, be careful while giving forward & backward slashes.
Below is the working code in my local system.
<iframe src="file:///C:\Users\105800\Downloads\Systems - Casinolink - Slot.pdf" style="height:700px; width:500px;" frameborder="0.3"></iframe>
Hope, it helps.
Thanks.