Target framework changed to 3.5 for backward compatibility with some slight changes for that to work.

DocX/Container.cs

 
         public virtual void InsertAtBookmark(string toInsert, string bookmarkName)
         {
-            if (String.IsNullOrWhiteSpace(bookmarkName))
+            if (bookmarkName.IsNullOrWhiteSpace())
                 throw new ArgumentException("bookmark cannot be null or empty", "bookmarkName");
 
             var headerCollection = Document.Headers;

DocX/DocX.csproj

     <AppDesignerFolder>Properties</AppDesignerFolder>
     <RootNamespace>Novacode</RootNamespace>
     <AssemblyName>DocX</AssemblyName>
-    <TargetFrameworkVersion>v4.0</TargetFrameworkVersion>
+    <TargetFrameworkVersion>v3.5</TargetFrameworkVersion>
     <FileAlignment>512</FileAlignment>
     <SccProjectName>SAK</SccProjectName>
     <SccLocalPath>SAK</SccLocalPath>

DocX/HelperFunctions.cs

     {
         public const string DOCUMENT_DOCUMENTTYPE = "application/vnd.openxmlformats-officedocument.wordprocessingml.document.main+xml";
         public const string TEMPLATE_DOCUMENTTYPE = "application/vnd.openxmlformats-officedocument.wordprocessingml.template.main+xml";
+
+            public static bool IsNullOrWhiteSpace(this string value)
+            {
+                if (value == null) return true;
+                return string.IsNullOrEmpty(value.Trim());
+            }
+
         /// <summary>
         /// Checks whether 'toCheck' has all children that 'desired' has and values of 'val' attributes are the same
         /// </summary>

Documentation/Documentation.shfbproj

     <Language>en-US</Language>
     <HelpFileFormat>HtmlHelp1</HelpFileFormat>
     <IndentHtml>False</IndentHtml>
-    <FrameworkVersion>.NET Framework 4.0</FrameworkVersion>
+    <FrameworkVersion>.NET Framework 3.5</FrameworkVersion>
     <KeepLogFile>True</KeepLogFile>
     <DisableCodeBlockComponent>False</DisableCodeBlockComponent>
     <CppCommentsFixup>False</CppCommentsFixup>