Envision Image Library uses FreeImage internally.

Should Envision ImageEn be open-sourced?

Envision Image Library should be an considered as a closed-source wrapper using PDFium and FreeImage.dll (hidden inside Envision's RES file).

Envision Logo - Photocredit: source.

Embedded FreeImage.DLLs found in Envision:
enfreeimage32.RES
enfreeimage64.RES

Embedded PDFium DLLs found in Envision:
enpdfium32.RES
enpdfium64.RES

If you are using Envision, consider using FreeImage and open-source PDFium.