Symptoms

In case multiple security providers/issuers configured Create PDF from URL method of PDF converter SMO produces an empty form.
 

Diagnoses

It is impossible to generate PDF from smartform URL when multiple security providers are enabled. This is caused by the login page, where it is necessary to select label, which is not something being done by the PDF Converter Service by default.
As a workaround to this, it is possible to configure smartforms as anonymous but this is not always an acceptable solution.
To address this in a long run there is an existing feature request to add an ability to set default label for PDF converter service.
 

Resolution

Configure SmartForm(s) targeted by PDF converter with Anonymous Access advanced property enabled as a workaround to this issue. See screenshot below which demonstrates relevant setting:

The Anonymous Access View or Form setting allows a View or Form to be marked as Anonymous. This allows the View or Form to be executed as the Application Pool Account user inside a site that normally requires authentication. The Anonymous Access View or Form setting is more secure and removes the need for a second site configured for anonymous access.

Refer to the "Anonymous Views/Forms" in official K2 documentation for details about this setting.