We are upgrading from iText 2.1.7 to the iText 7.1.13 latest version. we are not using maven. Its a existing project so we cant change it to maven. Just directly adding the jar. For the iText 2.1.7 we have iText-2.1.7 jar, but now I can't seem to find the main iText-7.1.13 jar. where can I get iText jar. The zip files here provide the supporting jar but does not contain - iText jar. https://github.com/itext/itext7/releases/tag/7.1.13
For iText 7 there is not one single jar, it is split up in several modules. You need all the jars in the ...-only-jars.zip on the page your provided. Ok, maybe not all the jars, if you don't use certain features, but at least io, kernel, layout and maybe other modules too, depending on which features you need. For example if you don't use barcodes then you don't need the barcodes jar.
Also, iText 7 is a complete rewrite, and the API is not backwards compatible. Which means that you may need to rewrite parts of your code.