PDF to Base64 vs PDF to Data URI
Compare a raw Base64 string and a full data URI for a PDF — what each one contains, when a browser needs the prefix, and when the raw string is enough.
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The Short Answer
A raw Base64 string is only the encoded bytes. A data URI is that same string wrapped in a prefix — data:application/pdf;base64, — that tells whatever reads it both the file type and the encoding used. The data after the comma is identical in both cases.
When You Need a Data URI
A browser, an <iframe>, an <embed> element, or a CSS background property cannot guess what a plain string represents. The data URI's prefix supplies the missing context, so the browser knows to render the bytes as a PDF instead of treating them as random text.
Anywhere a URL is expected — an src or href attribute, for instance — a data URI works because it is itself a valid URL scheme.
When Raw Base64 Is Enough
Inside a JSON API field, a database column, or a config file, the content type is usually carried by a separate field, so the prefix would be redundant — and pasting it into a field meant for raw Base64 corrupts the value. See how to use a Base64 PDF in a JSON API request for how those fields are usually structured.
Converting Between the Two
Adding the prefix data:application/pdf;base64, in front of a raw Base64 string turns it into a data URI. Going the other way, removing everything up to and including the first comma turns a data URI back into the raw string.
Either form encodes and decodes the same way underneath — see how Base64 encoding works for the mechanics, and the Base64 and PDF glossary for quick definitions of both terms.