Hi Liat,
We tested the use of the PDF 4.2 with the existing 2.x projects and it works fine as expected. But I would like to recommend you not to use multiple versions of Syncfusion in the same application. Since I fear this may create versioning conflicts and licensing issues.
I would like to recommend you to upgrade to our latest version 4.2.0.37 from the link shown below.
http://www.syncfusion.com/Downloads/latestversion.aspx
This version has a lot of fixes and features which will help improve your application in all aspects.
Thank you for your interest in Syncfusion Essential Studio.
Regards,
Ajish.