Hi Mithun,
Thanks for your inquiry.
I did an initial test with FreeSpire.PDF v5.1.0 and the latest FreeSpire.PDF 6.2.0 but didn't encounter the issue you mentioned. Kindly note that our Free version is totally free without any warning message. Please make sure you have completely removed the commercial version of Spire.PDF from your project.
Here I uploaded my test project for your reference, please run it directly on your side.
http://www.e-iceblue.com/downloads/demo/21702Demo.zip
Sincerely,
Rachel
E-iceblue support team