diff --git a/frontend/messages/de.json b/frontend/messages/de.json
index 1ded45d1..41278bc6 100644
--- a/frontend/messages/de.json
+++ b/frontend/messages/de.json
@@ -11,6 +11,7 @@
 	"error_file_not_found": "Die Datei konnte im Speicher nicht gefunden werden.",
 	"error_file_upload_failed": "Die Datei konnte nicht hochgeladen werden.",
 	"error_unsupported_file_type": "Dieses Dateiformat wird nicht unterstützt.",
+	"error_file_too_large": "Die Datei ist zu groß (max. 50 MB).",
 	"error_user_not_found": "Der Benutzer wurde nicht gefunden.",
 	"error_import_already_running": "Ein Import läuft bereits. Bitte warten Sie, bis dieser abgeschlossen ist.",
 	"error_unauthorized": "Sie sind nicht angemeldet.",
diff --git a/frontend/messages/en.json b/frontend/messages/en.json
index 98e0faab..1c6e4b6e 100644
--- a/frontend/messages/en.json
+++ b/frontend/messages/en.json
@@ -11,6 +11,7 @@
 	"error_file_not_found": "The file could not be found in storage.",
 	"error_file_upload_failed": "The file could not be uploaded.",
 	"error_unsupported_file_type": "This file format is not supported.",
+	"error_file_too_large": "The file is too large (max. 50 MB).",
 	"error_user_not_found": "User not found.",
 	"error_import_already_running": "An import is already running. Please wait for it to finish.",
 	"error_unauthorized": "You are not logged in.",
diff --git a/frontend/messages/es.json b/frontend/messages/es.json
index 1a4e11df..254e17cd 100644
--- a/frontend/messages/es.json
+++ b/frontend/messages/es.json
@@ -11,6 +11,7 @@
 	"error_file_not_found": "El archivo no pudo encontrarse en el almacenamiento.",
 	"error_file_upload_failed": "No se pudo subir el archivo.",
 	"error_unsupported_file_type": "Este formato de archivo no está admitido.",
+	"error_file_too_large": "El archivo es demasiado grande (máx. 50 MB).",
 	"error_user_not_found": "Usuario no encontrado.",
 	"error_import_already_running": "Ya hay una importación en curso. Por favor, espere a que finalice.",
 	"error_unauthorized": "No ha iniciado sesión.",
diff --git a/frontend/src/lib/components/document/DocumentEditLayout.svelte b/frontend/src/lib/components/document/DocumentEditLayout.svelte
index 61024d00..04bb203c 100644
--- a/frontend/src/lib/components/document/DocumentEditLayout.svelte
+++ b/frontend/src/lib/components/document/DocumentEditLayout.svelte
@@ -6,6 +6,7 @@ import type { Snippet } from 'svelte';
 import { createFileLoader } from '$lib/hooks/useFileLoader.svelte';
 import { m } from '$lib/paraglide/messages.js';
 import { countRequiredFilled } from '$lib/utils/requiredFields';
+import { validateFile } from '$lib/utils/validateFile';
 import DocumentViewer from '$lib/components/DocumentViewer.svelte';
 import UploadZone from '$lib/components/document/UploadZone.svelte';
 import WhoWhenSection from '$lib/components/document/WhoWhenSection.svelte';
@@ -113,6 +114,15 @@ function cancelUpload() {
 async function handleReplaceFile(e: Event) {
 	const file = (e.currentTarget as HTMLInputElement).files?.[0];
 	if (!file) return;
+	const validationError = validateFile(file);
+	if (validationError === 'type') {
+		uploadError = m.error_unsupported_file_type();
+		return;
+	}
+	if (validationError === 'size') {
+		uploadError = m.error_file_too_large();
+		return;
+	}
 	await handleFile(file);
 }
 </script>
diff --git a/frontend/src/lib/utils/validateFile.spec.ts b/frontend/src/lib/utils/validateFile.spec.ts
new file mode 100644
index 00000000..7ec2838b
--- /dev/null
+++ b/frontend/src/lib/utils/validateFile.spec.ts
@@ -0,0 +1,36 @@
+import { describe, it, expect } from 'vitest';
+import { validateFile, MAX_SIZE_BYTES } from './validateFile';
+
+function makeFile(type: string, size: number): File {
+	return new File(['x'.repeat(Math.min(size, 100))], 'test.file', { type });
+}
+
+describe('validateFile', () => {
+	it('returns null for a valid PDF under 50 MB', () => {
+		const file = makeFile('application/pdf', 1024);
+		expect(validateFile(file)).toBeNull();
+	});
+
+	it('returns null for a valid JPEG', () => {
+		expect(validateFile(makeFile('image/jpeg', 1024))).toBeNull();
+	});
+
+	it('returns null for a valid PNG', () => {
+		expect(validateFile(makeFile('image/png', 1024))).toBeNull();
+	});
+
+	it('returns null for a valid TIFF', () => {
+		expect(validateFile(makeFile('image/tiff', 1024))).toBeNull();
+	});
+
+	it('returns "type" for an unsupported MIME type', () => {
+		const file = makeFile('text/plain', 100);
+		expect(validateFile(file)).toBe('type');
+	});
+
+	it('returns "size" for a file exceeding 50 MB', () => {
+		const oversized = new File(['x'], 'big.pdf', { type: 'application/pdf' });
+		Object.defineProperty(oversized, 'size', { value: MAX_SIZE_BYTES + 1 });
+		expect(validateFile(oversized)).toBe('size');
+	});
+});
diff --git a/frontend/src/lib/utils/validateFile.ts b/frontend/src/lib/utils/validateFile.ts
new file mode 100644
index 00000000..77d8c570
--- /dev/null
+++ b/frontend/src/lib/utils/validateFile.ts
@@ -0,0 +1,10 @@
+export const ALLOWED_TYPES = new Set(['application/pdf', 'image/jpeg', 'image/png', 'image/tiff']);
+export const MAX_SIZE_BYTES = 50 * 1024 * 1024;
+
+export type FileValidationError = 'type' | 'size';
+
+export function validateFile(file: File): FileValidationError | null {
+	if (!ALLOWED_TYPES.has(file.type)) return 'type';
+	if (file.size > MAX_SIZE_BYTES) return 'size';
+	return null;
+}