
I know, Sigils the same.
Another reason not to use Acrobat Reader

Although I suspect other PDF readers may be similar.
BTW:
it seems that Office 2010 doesn't hold locks on open files either, I can convert a DOCX that's open in Word that I haven't saved - and then puzzle why the changes I just made in the DOCX aren't in the EPUB

I couldn't do that with Office 2007 - calibre would throw an IO error. Trying to get a straight answer from MS on such issues is like extracting hens teeth - no glitz, no glamour - no answers forthcoming.
BR