There is no way to get those old versions and in this case, it won't do you any good anyway.
CS6 was the last InDesign version that could open PM files and that was limited to PM6 or later. An additional complication is that in order to open the PM5 file in PM 6, 6.5 or 7 you would have to open it on the same platform it was created on.
That means if it was Mac file (and without a file extension, I would guess that to be the case) you're going to have to find someone with a very old Mac that has PM 6 or later installed.
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