Updated copy on the page.

Added unit tests.
Added error when file is too big.

app/main/views/platform_admin.py

@@ -280,6 +280,14 @@ def letter_validation_preview(from_platform_admin):
             ), 400
 
         try:
+            if len(pdf_file.read()) > (2 * 1024 * 1024):
+                return render_template(
+                    view_location,
+                    form=form,
+                    message="File must be less than 2MB",
+                    pages=pages, result=result
+                ), 400
+            pdf_file.seek(0)
             response = validate_letter(pdf_file)
             response.raise_for_status()
             if response.status_code == 200:

app/templates/views/letter-validation-preview.html

@@ -5,6 +5,11 @@
 {% from "components/page-footer.html" import page_footer %}
 
 {% from "components/form.html" import form_wrapper %}
+
+{% block service_page_title %}
+  Letter validation preview
+{% endblock %}
+
 {% block maincolumn_content %}
 
 <div class="grid-row">
@@ -15,14 +20,29 @@
       {{ banner(message, 'dangerous')}}
     {% endif %}
 
-    <h1 class="heading-large">Letter Validation Preview</h1>
+      <h1 class="heading-large">Letter validation preview</h1>
     <div class="bottom-gutter">
      {{ file_upload(
           form.file,
-          action = url_for('main.services_letter_validation_preview', service_id=current_service.id),
-          button_text='Upload a PDF document',
+          action = url_for('main.service_letter_validation_preview', service_id=current_service.id),
+          button_text='Choose a file',
       )}}
     </div>
+      {%if not pages %}
+        <div>
+        <h1 class="heading-medium">Check your file meets the letter specification </h1>
+        <p>Your file must be:
+        <li>a PDF</li>
+        <li>no more than 10 pages long</li>
+        <li>less than 2 MB</li>
+        </p>
+        <p>The content of your letter mut appear inside the printable area.</p>
+        <p>
+        <a href="https://docs.notifications.service.gov.uk/documentation/images/notify-pdf-letter-spec-v2.3.pdf">
+            Download the letter specification</a> for more information.
+        </p>
+        </div>
+    {% endif%}
   </div>
   <div class="column-whole template-container" >
 

tests/app/main/views/test_platform_admin.py

@@ -761,6 +761,36 @@ def test_platform_admin_submit_empty_returned_letters(mocker, client, platform_a
     assert "Can’t be empty" in response.get_data(as_text=True)
 
 
+def test_service_letter_validation_preview_renders_correctly(
+        client_request,
+        mock_has_no_jobs
+
+):
+    page = client_request.get('main.service_letter_validation_preview', service_id="service_1")
+
+    assert page.find('h1').text.strip() == "Letter validation preview"
+    assert page.find_all('input', class_='file-upload-field')
+
+
+def test_service_letter_validation_preview_returns_400_if_file_is_too_big(
+        client_request,
+        mock_has_no_jobs,
+        mocker
+
+):
+    with open('tests/test_pdf_files/big.pdf', 'rb') as file:
+        page = client_request.post('main.service_letter_validation_preview', service_id="service_1",
+                                   _data=dict(
+                                       pdf_file=file,
+                                   ),
+                                   content_type='multipart/form-data',
+                                   _follow_redirects=True)
+
+    assert page.find('h1').text.strip() == "Letter validation preview"
+    assert page.find_all('input', class_='file-upload-field')
+    page.find('span', class_='error-message').text.strip() == "File must be less than 2MB"
+
+
 def test_letter_validation_preview_renders_correctly(mocker, client, platform_admin_user):
     mock_get_user(mocker, user=platform_admin_user)
     client.login(platform_admin_user)