Another question, have you used any library to convert PDFs to base64 in suitescript. I can see this, https://www.npmjs.com/package/pdf-to-base64 , but not sure if I can use this in suitescript.

No, but I'm not sure why you'd want to use that? If you save the PDF in the fileCabinet you just do getContents() on the file...

I've done rudimentary tests and confirmed that

pdf-lib

worked in suitescript

PDF is automagically base64 converted via N/file.getContets() according to SA: 99116 (What NElliot is saying). There's also N/encode so no external libs should be needed.