Let us assume the pdf is at url. To convert this pdf to base64 we will download it as blob and the convert it to base64 url and then remove the url part of the base64 url.

1. Download

   const response = await fetch(url);
   const fileBlob = await response.blob();

2. If you want to convert local file

   let selectedFile = document.getElementById("inputFile").files;
   let fileBlob = selectedFile[0];

3. Create a FileReader and convert to dataurl / base64 url

   const reader = new FileReader();
   reader.readAsDataURL(fileBlob);

4. Remove the url part of base64 url

   reader.onload = (evt) => {
   	const base64data = evt.target.result.replace(/^data:.+;base64,/, "");
   
   	// do anything you want
   	console.log(base64data);
   };

We are doing things inside the callback in reader.onload because it takes time to download the pdf.