diff --git a/Api.SeaHavenIndustries/Controllers/WorkOrderCompletionController.cs b/Api.SeaHavenIndustries/Controllers/WorkOrderCompletionController.cs
index bf90361..88837ef 100644
--- a/Api.SeaHavenIndustries/Controllers/WorkOrderCompletionController.cs
+++ b/Api.SeaHavenIndustries/Controllers/WorkOrderCompletionController.cs
@@ -5,6 +5,7 @@ using Microsoft.AspNetCore.Authorization;
 using Microsoft.AspNetCore.Mvc;
 using SeaHaven.Services.DTOs;
 using SeaHaven.Services.Exceptions;
+using SeaHaven.Services.Helpers;
 using SeaHaven.Services.Interfaces;
 using System.Security.Claims;
 
@@ -109,6 +110,7 @@ namespace Api.SeaHavenIndustries.Controllers
             string? fileUrl = null;
             try
             {
+                WorkOrderCompletionDocFileRules.EnsureAllowed(file);
                 var actorId = User.FindFirstValue(ClaimTypes.NameIdentifier);
                 await _workOrderCompletionService.EnsureCanUploadCompletionDocAsync(id, User, actorId);
 
diff --git a/SeaHaven.Services/Helpers/WorkOrderCompletionDocFileRules.cs b/SeaHaven.Services/Helpers/WorkOrderCompletionDocFileRules.cs
new file mode 100644
index 0000000..5d3d4d7
--- /dev/null
+++ b/SeaHaven.Services/Helpers/WorkOrderCompletionDocFileRules.cs
@@ -0,0 +1,70 @@
+using Microsoft.AspNetCore.Http;
+
+namespace SeaHaven.Services.Helpers
+{
+    /// <summary>SH-337 file type allowlist for work-order completion documents.</summary>
+    public static class WorkOrderCompletionDocFileRules
+    {
+        private const string PdfContentType = "application/pdf";
+
+        private static readonly HashSet<string> AllowedExtensions =
+            new(StringComparer.OrdinalIgnoreCase) { ".pdf" };
+
+        // A browser sets the multipart part's Content-Type from File.type, which the OS
+        // sometimes leaves empty for a PDF. The completion-doc dialog already accepts
+        // that case on the client (a .pdf name with no type passes), so rejecting it
+        // here would fail uploads that work today. The %PDF- signature is the real gate.
+        private static readonly HashSet<string> UndeterminedContentTypes =
+            new(StringComparer.OrdinalIgnoreCase) { "application/octet-stream" };
+
+        public static bool IsAllowed(IFormFile file)
+        {
+            if (file == null || file.Length <= 0)
+                return false;
+
+            var declaredType = (file.ContentType ?? string.Empty).Trim();
+            if (!string.IsNullOrWhiteSpace(declaredType)
+                && !declaredType.Equals(PdfContentType, StringComparison.OrdinalIgnoreCase)
+                && !UndeterminedContentTypes.Contains(declaredType))
+            {
+                return false;
+            }
+
+            var extension = Path.GetExtension(file.FileName ?? string.Empty);
+            if (string.IsNullOrWhiteSpace(extension) || !AllowedExtensions.Contains(extension))
+                return false;
+
+            try
+            {
+                using var stream = file.OpenReadStream();
+                var headerLength = (int)Math.Min(Math.Max(file.Length, 0), 64);
+                if (headerLength == 0)
+                    return false;
+
+                var header = new byte[headerLength];
+                var read = stream.Read(header, 0, header.Length);
+                if (read <= 0)
+                    return false;
+
+                if (read < header.Length)
+                    Array.Resize(ref header, read);
+
+                return WorkOrderMediaFileRules.MatchesSignature(PdfContentType, header);
+            }
+            catch
+            {
+                return false;
+            }
+        }
+
+        public static void EnsureAllowed(IFormFile file)
+        {
+            if (!IsAllowed(file))
+            {
+                throw new Exceptions.WorkOrderBoardValidationException(
+                    "UnsupportedMediaType",
+                    "The completion document must be a PDF file.");
+            }
+        }
+    }
+}
diff --git a/SeaHaven.Services/Helpers/WorkOrderMediaFileRules.cs b/SeaHaven.Services/Helpers/WorkOrderMediaFileRules.cs
index 68381d9..d483ba9 100644
--- a/SeaHaven.Services/Helpers/WorkOrderMediaFileRules.cs
+++ b/SeaHaven.Services/Helpers/WorkOrderMediaFileRules.cs
@@ -99,6 +99,14 @@ namespace SeaHaven.Services.Helpers
                 return bytes.Length >= 3 && bytes[0] == 0xFF && bytes[1] == 0xD8 && bytes[2] == 0xFF;
             }
 
+            if (contentType.Equals("application/pdf", StringComparison.OrdinalIgnoreCase))
+            {
+                // %PDF-
+                return bytes.Length >= 5
+                    && bytes[0] == 0x25 && bytes[1] == 0x50 && bytes[2] == 0x44
+                    && bytes[3] == 0x46 && bytes[4] == 0x2D;
+            }
+
             if (contentType.Equals("video/mp4", StringComparison.OrdinalIgnoreCase)
                 || contentType.Equals("video/quicktime", StringComparison.OrdinalIgnoreCase))
             {
diff --git a/SeaHavenIndustries.Tests/WorkOrderPhase6Tests.cs b/SeaHavenIndustries.Tests/WorkOrderPhase6Tests.cs
index b770313..0c7a906 100644
--- a/SeaHavenIndustries.Tests/WorkOrderPhase6Tests.cs
+++ b/SeaHavenIndustries.Tests/WorkOrderPhase6Tests.cs
@@ -1732,6 +1732,78 @@ public class WorkOrderMediaServiceTests
     }
 }
 
+public class WorkOrderCompletionDocFileRulesTests
+{
+    private static readonly byte[] Pdf = Encoding.ASCII.GetBytes("%PDF-1.7\n1 0 obj\n");
+
+    private static FormFile FormFile(byte[] bytes, string fileName, string contentType)
+        => new(new MemoryStream(bytes), 0, bytes.Length, "file", fileName)
+        {
+            Headers = new HeaderDictionary(),
+            ContentType = contentType
+        };
+
+    [Fact]
+    public void IsAllowed_ValidPdf_ReturnsTrue()
+    {
+        Assert.True(WorkOrderCompletionDocFileRules.IsAllowed(FormFile(Pdf, "completion.pdf", "application/pdf")));
+    }
+
+    [Fact]
+    public void IsAllowed_PdfWithUndeterminedContentType_ReturnsTrue()
+    {
+        // The browser leaves File.type empty when the OS cannot determine it, and the
+        // completion-doc dialog already lets that through on a .pdf name. Rejecting it
+        // would break uploads that work today.
+        Assert.True(WorkOrderCompletionDocFileRules.IsAllowed(FormFile(Pdf, "completion.pdf", "")));
+        Assert.True(WorkOrderCompletionDocFileRules.IsAllowed(FormFile(Pdf, "completion.pdf", "application/octet-stream")));
+    }
+
+    [Fact]
+    public void IsAllowed_PdfExtensionWithNonPdfContent_ReturnsFalse()
+    {
+        var html = Encoding.UTF8.GetBytes("<html>not a pdf</html>");
+        Assert.False(WorkOrderCompletionDocFileRules.IsAllowed(FormFile(html, "completion.pdf", "application/pdf")));
+    }
+
+    [Fact]
+    public void IsAllowed_PdfContentUnderDisallowedExtension_ReturnsFalse()
+    {
+        Assert.False(WorkOrderCompletionDocFileRules.IsAllowed(FormFile(Pdf, "completion.exe", "application/pdf")));
+        Assert.False(WorkOrderCompletionDocFileRules.IsAllowed(FormFile(Pdf, "completion", "application/pdf")));
+    }
+
+    [Fact]
+    public void IsAllowed_DisallowedContentType_ReturnsFalse()
+    {
+        var jpeg = new byte[] { 0xFF, 0xD8, 0xFF, 0xE0, 0x00, 0x10 };
+        Assert.False(WorkOrderCompletionDocFileRules.IsAllowed(FormFile(jpeg, "photo.jpg", "image/jpeg")));
+        Assert.False(WorkOrderCompletionDocFileRules.IsAllowed(FormFile(Pdf, "completion.pdf", "text/html")));
+    }
+
+    [Fact]
+    public void IsAllowed_EmptyFile_ReturnsFalse()
+    {
+        Assert.False(WorkOrderCompletionDocFileRules.IsAllowed(FormFile(Array.Empty<byte>(), "completion.pdf", "application/pdf")));
+    }
+
+    [Fact]
+    public void EnsureAllowed_RejectedFile_ThrowsUnsupportedMediaType()
+    {
+        var html = Encoding.UTF8.GetBytes("<html>not a pdf</html>");
+        var ex = Assert.Throws<WorkOrderBoardValidationException>(
+            () => WorkOrderCompletionDocFileRules.EnsureAllowed(FormFile(html, "completion.pdf", "application/pdf")));
+        Assert.Equal("UnsupportedMediaType", ex.Code);
+    }
+
+    [Fact]
+    public void MediaRules_StillRejectPdf()
+    {
+        // The completion-doc allowlist must not widen the SH-116 media allowlist.
+        Assert.False(WorkOrderMediaFileRules.IsAllowed(FormFile(Pdf, "completion.pdf", "application/pdf")));
+    }
+}
+
 public class WorkOrderMediaFileRulesTests
 {
     private static FormFile FormFile(byte[] bytes, string fileName, string contentType)