If you try to open a PDF document over a network connection (e.g. via RDP or Citrix) some more recent versions of sign pro PDF will freeze with a revolving blue circle.
Until this bug is fixed the current workaround is to install an older version of the app.
The recommended version is 184.108.40.206.
To obtain this please raise a support ticket using the "Submit a request" link at the top of this page.