A lot of PDF readers are vulnerable to PDFex. Since SumatraPDF was not listed by the researchers (you can find about it at https://pdf-insecurity.org/index.html ) I hope somebody (SumatraPDF’s developer or a forum admin) can elaborate on it since lastest SumatraPDF release was 3 years ago. Thank you.
SumatraPDF is unlikely to be affected by any such vulnerabilities (with one or two exceptions) exploiting the PDF Readers that support adobe’s security vulnerabilities to scripting.
In short SumatraPDF is not affected by such exploits because it does not run any code for “Signatures” nor “Forms” etc. etc.
SumatraPDF is not listed because it can not be affected.