It is not a legal thing. The only time I ever see it used is if the document is intended to be printed and you know that the document will be printed two-sided. Most often these days, PDF documents are viewed online, and they are not always printed two-sided. (You would want a blank page in order to start a new chapter on an odd page.)
If you know how the PDF is intended to be used, and it will likely be viewed online, or printed in chunks (as in, a page here or there), printed one-sided, or your tool automatically numbers chapters so that they start on an odd page, then you can forgo the "intentionally left blank" statement. Most people these days are smart enough to figure out when there is a blank page somewhere...
(Of course, your mileage may vary, but I will say, it's probably been maybe the late 80's since I've had anyone on my team add this statement in a manual...)