Pdf file size

Save it to your hard drive then look at its file properties.

If what I understand is correct,
And the OS is windows, just pointing the mouse over the file for a few seconds will tell you the file size.
Or are you talking about something more complciated situation?

Use Windows Explorer, it gives a list of the files in your folders including their size.

Right-click the file (must be on your hard drive) -> Properties displays the file size. Or pointing the mouse over the file for a few seconds.