I got a note from client. Adobe Reader update seems to be installed automatically, which delivers a 32-bit version to Windows users. That breaks FileMaker looking for a 64-bit browser plugin to display PDF.
Solution may be to disable automatic updates until Adobe fixes that.
And if needed uninstall Adobe Reader and install an older offline installer.
Alternatively you can always use plugins (DynaPDF.RenderPage), command line tools or web services to render PDF to picture and show the picture.