How to add item transform to VS2012 .proj msbuild

Based off this answer describing an item transform to convert image files from jpg to png, I made an item transform that converts .docx file to .pdf. When I call it from my projectname.proj build file I get this error message:

Error   1   The condition " '%(Extension)' == '.docx' " on the
"WordToPdf" target has a reference to item metadata. References to item 
metadata are not allowed in target conditions unless they are part of an item
transform. [project path]\.build\WordToPdf.Tasks.target 7   9

How can I make this work?

Here's my target:

    <?xml version="1.0" encoding="utf-8" ?>
    <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
      <!-- $Id$ -->
      <Target Name="WordToPdf"
            Inputs="@(Content)"
            Outputs="@(Content -> '%(RootDir)%(Directory)%(Filename).pdf' )"
            Condition=" '%(Extension)' == '.docx' ">
        <ItemGroup>
          <Sublist Include="@(Content)" Condition=" '%(Extension)' == '.docx' " />
        </ItemGroup>
        <PropertyGroup>
          <PowerShellExe Condition=" '$(PowerShellExe)'=='' ">%WINDIR%\System32\WindowsPowerShell\v1.0\powershell.exe</PowerShellExe>
          <ScriptLocation Condition=" '$(ScriptLocation)'=='' ">.\WordToPdf.ps1</ScriptLocation>
        </PropertyGroup>

        <Exec Command="$(PowerShellExe) -NonInteractive -executionpolicy Unrestricted -command &quot;&amp; {&amp;&apos;$(ScriptLocation)&apos; -WordFilename &apos;/Input:%(Sublist.FullPath)&apos; }&quot;" />

        <Content Remove="@(Sublist)" />
        <Content Include="@(Sublist -> '%(RootDir)%(Directory)%(Filename).pdf' )" />
      </Target>
    </Project>

And call it from my projectname.proj file from the BeforeBuild target, as follows:

  <Import Project="$(MSBuildTasksPath)\WordToPdf.Tasks.target" />

  ....

  <Target Name="BeforeBuild">
    <CallTarget Targets="WordToPdf" />
  </Target>

Update

There was a bit more wrong with this target beyond just the workaround to get the transform to work. For future reference, here's the final working code:

<?xml version="1.0" encoding="utf-8" ?>
<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <!-- $Id$ -->
  <Target Name="WordToPdf"
      Inputs="@(ContentFiltered)"
      Outputs="@(ContentFiltered -> '%(RootDir)%(Directory)%(Filename).pdf' )"
      DependsOnTargets="FilterContent">
    <PropertyGroup>
      <PowerShellExe Condition=" '$(PowerShellExe)'=='' ">%WINDIR%\System32\WindowsPowerShell\v1.0\powershell.exe</PowerShellExe>
      <ScriptLocation Condition=" '$(ScriptLocation)'=='' ">.\WordToPdf.ps1</ScriptLocation>
    </PropertyGroup>

    <Exec Command="$(PowerShellExe) -NonInteractive -executionpolicy Unrestricted -command &quot;&amp; {&amp;&apos;$(ScriptLocation)&apos; -WordFilename &apos;%(ContentFiltered.FullPath)&apos; }&quot;" />

    <ItemGroup>
      <Content Remove="@(ContentFiltered)" />
      <Content Include="@(ContentFiltered -> '%(RootDir)%(Directory)%(Filename).pdf' )" />
    </ItemGroup>
  </Target>
  <!-- New target to pre-filter list -->
  <Target Name="FilterContent">
    <ItemGroup>
      <ContentFiltered Include="@(Content)" Condition="'%(Extension)' == '.docx'" />
    </ItemGroup>
  </Target>

</Project>

And, for anyone who got here searching for a "msbuild word to pdf item transform" here's my powershell script:

Param(
  [string]$WordFilename
)
$Word = New-Object -comobject Word.Application
$Doc=$Word.Documents.Open($WordFilename)
$Doc.saveas([ref](($WordFilename).replace("docx","pdf")), [ref]17)
$Doc.close()

Add the .docx file to your project, as content - copy if newer, and the pdf file will be created if the docx file is newer, and then the pdf file is copied to its output location.

You can workaround it by creating separate target, which pre-filters items based on your condition - actually, creates new list.

Here is sample - notice new DependsOnTargets tag and changed ContentFiltered name:

  <Target Name="WordToPdf"
        Inputs="@(ContentFiltered)"
        Outputs="@(ContentFiltered -> '%(RootDir)%(Directory)%(Filename).pdf' )"
        DependsOnTargets="FilterContent">
    <!-- 
      ...
      your actual target body here 
      ...
      -->
  </Target>

  <!-- New target to pre-filter list -->
  <Target Name="FilterContent">
    <ItemGroup>
      <ContentFiltered Include="@(Content)" Condition="'%(Extension)' == '.docx'" />
    </ItemGroup>
  </Target>