Here is an quick example how to encode/decode a string to base64. This example works already on .net core 2.0 or above.
.Net Core PDF Library from HTML
If you need an PDF library, wich converts an HTML document to a PDF you need a library. There are a lot of libraries on the market, but only a few wich are free and open source. In my projects i used the DinkToPdf library, wich is a wrapper for the webkit engine (the chrome engine) for c#/.Net Core. The library needs as dependency the wkhtmltopdf library, wich you need to add manually.