Base64 to PDF Converter

How do I turn a Base64 string back into a PDF file?

Supply the Base64 content and decode it to get a normal .pdf download. Both raw Base64 and a full data:application/pdf;base64 URL are accepted, whitespace and line breaks are stripped automatically, and the decoded bytes are checked for the PDF file signature so you find out immediately whether the payload was really a PDF. Decoding happens locally.

Three things break decoding more often than anything else: a data URI prefix that was never stripped, whitespace or newlines left in the string, and URL-safe Base64 (- and _ instead of + and /). Each snippet below handles all three.

function base64ToPdfBlob(input) {
  let payload = input.trim()
  if (payload.startsWith("data:")) payload = payload.slice(payload.indexOf(",") + 1)
  payload = payload.replace(/\s+/g, "").replace(/-/g, "+").replace(/_/g, "/")

  const binary = atob(payload)
  const bytes = new Uint8Array(binary.length)
  for (let i = 0; i < binary.length; i++) bytes[i] = binary.charCodeAt(i)

  // A real PDF always starts with the five bytes %PDF-
  if (binary.slice(0, 5) !== "%PDF-") throw new Error("Not a PDF")
  return new Blob([bytes], { type: "application/pdf" })
}

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

const payload = input.replace(/^data:[^,]+,/, "").replace(/\s+/g, "")
const bytes = Buffer.from(payload, "base64")

if (bytes.subarray(0, 5).toString("latin1") !== "%PDF-") {
  throw new Error("Decoded bytes are not a PDF")
}
await writeFile("invoice.pdf", bytes)

import base64, re

payload = re.sub(r"^data:[^,]+,", "", raw.strip())
data = base64.b64decode(payload)

assert data[:5] == b"%PDF-", "Decoded bytes are not a PDF"
with open("invoice.pdf", "wb") as handle:
    handle.write(data)

sed -e "s|^data:[^,]*,||" invoice.b64 | base64 -d > invoice.pdf

# Confirm the first bytes really are a PDF header
head -c 5 invoice.pdf

<?php
$payload = preg_replace("/^data:[^,]+,/", "", trim($raw));
$bytes = base64_decode($payload, true);
if ($bytes === false || substr($bytes, 0, 5) !== "%PDF-") {
    throw new RuntimeException("Not a valid Base64 PDF");
}
file_put_contents("invoice.pdf", $bytes);

string payload = Regex.Replace(raw.Trim(), "^data:[^,]+,", "");
byte[] bytes = Convert.FromBase64String(payload);

if (Encoding.ASCII.GetString(bytes, 0, 5) != "%PDF-")
    throw new InvalidDataException("Decoded bytes are not a PDF");

File.WriteAllBytes("invoice.pdf", bytes);

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.