PDF to Base64 Converter

How do I convert a PDF to a Base64 string?

Select the PDF, choose the output shape you actually need — raw Base64, a data URI, a JSON envelope, XML, an HTML embed, object, iframe or download link, or a CSS url() value — and encode. You get back a file already in that form, ready to paste straight into the destination. Encoding is done by your browser, so the PDF never reaches a server.

The same conversion in six languages. All of them read the file as bytes and Base64-encode those bytes — the only thing that varies is which standard library call does it. Nothing here needs a package install.

const file = document.querySelector("input[type=file]").files[0]

const dataUri = await new Promise((resolve, reject) => {
  const reader = new FileReader()
  reader.onload = () => resolve(reader.result)
  reader.onerror = () => reject(reader.error)
  reader.readAsDataURL(file)
})

// "data:application/pdf;base64,JVBERi0xLjcK..."
const base64 = dataUri.slice(dataUri.indexOf(",") + 1)

const response = await fetch("/invoice.pdf")
const bytes = new Uint8Array(await response.arrayBuffer())

let binary = ""
for (let i = 0; i < bytes.length; i += 0x8000) {
  binary += String.fromCharCode(...bytes.subarray(i, i + 0x8000))
}
const base64 = btoa(binary)

import { readFile } from "node:fs/promises"

const bytes = await readFile("invoice.pdf")
const base64 = bytes.toString("base64")
const dataUri = "data:application/pdf;base64," + base64

import base64

with open("invoice.pdf", "rb") as handle:
    base64_pdf = base64.b64encode(handle.read()).decode("ascii")

# 76-character MIME lines, if the transport wants them
with open("invoice.pdf", "rb") as handle:
    wrapped = base64.encodebytes(handle.read()).decode("ascii")

# macOS
base64 -i invoice.pdf -o invoice.b64

# GNU coreutils (-w 0 stops it wrapping at 76 columns)
base64 -w 0 invoice.pdf > invoice.b64

curl -X POST https://api.example.com/documents \
  -H "Content-Type: application/json" \
  -d "{\"data\":\"$(cat invoice.b64)\"}"

<?php
$base64 = base64_encode(file_get_contents("invoice.pdf"));
$dataUri = "data:application/pdf;base64," . $base64;

byte[] bytes = File.ReadAllBytes("invoice.pdf");
string base64 = Convert.ToBase64String(bytes);

// 76-character lines separated by CRLF
string wrapped = Convert.ToBase64String(bytes, Base64FormattingOptions.InsertLineBreaks);

About a third bigger, always. Base64 turns every three bytes into four printable characters, so the encoded length is exactly ceil(bytes / 3) × 4, rounded up to a multiple of four with = padding. That is a 33.3% increase before any JSON escaping, and it is why a PDF that uploads fine as multipart form data is rejected when the same file is sent as an inline Base64 field.

Which request-body limits does that break?

Work backwards from the ceiling: divide the limit by 1.37 to get the largest PDF that still fits once encoded and wrapped in a JSON object.

• Vercel serverless function request body (4.5 MB) — fits a PDF of roughly 3.4 MB or less.
• AWS Lambda synchronous invoke payload (6 MB) — fits a PDF of roughly 4.5 MB or less.
• Amazon API Gateway payload (10 MB) — fits a PDF of roughly 7.5 MB or less.

Above those sizes, stop sending Base64. Upload the PDF as multipart/form-data, or hand the client a pre-signed URL and send only the resulting object key in your JSON. Base64 in a request body is a convenience for small documents, not a transfer mechanism for large ones.