---

title: "PDF validation" description: "Client-side PDF validation — magic bytes, MIME type, file size — before handing a file to the viewer or pdf.js fallback. Runs entirely in the browser." group: "Reference" order: 10

PDF validation

LoupePDF ships client-side validation utilities that run entirely in the browser. Use them before handing user-supplied files or URLs to the viewer.

Both are exported from @printwithsynergy/loupe-pdf/host.

validatePdfFile(file, maxBytes?)

Async. Checks a File object:

1. MIME type — must be application/pdf (or empty — some browsers don't populate MIME for drag-dropped files).
2. Magic bytes — first 5 bytes must be %PDF- (hex 25 50 44 46 2D).
3. Size — must be ≤ maxBytes (default: 50 MB).

import { validatePdfFile } from "@printwithsynergy/loupe-pdf/host";

const result = await validatePdfFile(file);
if (!result.valid) {
  console.error(result.error);
}

Pass a custom limit:

const result = await validatePdfFile(file, 100 * 1024 * 1024); // 100 MB

validatePdfUrl(url)

Synchronous. Validates URL format only — does not fetch or inspect headers.

Checks:

1. String is non-empty after trimming.
2. Parses as a valid URL.
3. Protocol is http:, https:, or blob:.

import { validatePdfUrl } from "@printwithsynergy/loupe-pdf/host";

const result = validatePdfUrl(draftUrl);
if (!result.valid) {
  setError(result.error);
  return;
}

PdfValidationResult

Both functions return the same shape:

interface PdfValidationResult {
  valid: boolean;
  error?: string; // Human-readable message when valid is false
}

Built-in usage

<LoupePDFDemo> calls both validators automatically — file uploads go through validatePdfFile and URL submissions go through validatePdfUrl. The maxFileSize prop on LoupePDFDemo is forwarded to validatePdfFile.

Security note

These checks are client-side only and meant to catch user mistakes early (wrong file type, oversized files, malformed URLs). They are not a security boundary — a determined user can bypass them. Always validate and sanitise uploads on the server side when applicable.