Is ambiguous in the namespace error

In my program, I installed a library Spire.Pdf using Package Manager.
It was working fine, but one day I started getting a compile error “‘PdfDocument’ is ambiguous in the namespace ‘Spire.Pdf’”.
This is happening where I am using Assign activity:
Assign: [pdf] = [new Spire.Pdf.PdfDocument()]

Variable pdf is of type Spire.Pdf.PdfDocument.

I am using the full name Spire.Pdf.PdfDocument instead of just PdfDocument, yet the compiler complains that it is ambiguous.
How could I fix this?