I modified ILMerge command to the following:
ilmerge D:\abc\RemittanceForm.exe D:\abc\PdfFileWriter.dll /out:D:\out\MyWinApp.exe /target:winexe /ndebug /log:D:\out\aaa.txt /targetplatform:"v4,C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.0"
Thank you @Magnus Grindal Bakken