fm2015: When I choose Publish html5, html generates *but* an image overwrites an open PDF

(Using FM2015 13.0.5.547 on a Windows10 machine)

I have *no* idea why this happens.

I have a book that I print to PDF and also publish to html5.

The published html5 includes an icon to open a pdf location (to view the 'original' pdf) -- let's call that PDF "Foo.pdf".

I found that if I had Foo.pdf open in Acrobat while I was publishing my html5, an image from the fifth doc in my book would overwrite the Foo.pdf. (That is, on-screen I would see the image in Acrobat for foo.pdf. Weird. Close the pdf without saving and reopen --- image is there. So, it's overwriting and saving the pdf.)

Fine, I'm under deadline, I can deal with that -- just don't have the pdf open (and figure it out later).

Well... I just generated html5 again and... an image from doc7 overwrites a *different* pdf that I had open in Acrobat.

SO... does anybody have any idea why this could be happening?

Thanks