%CSP.StreamServer not fully downloading file

Hi

I have a few Zen pages, one of which is implementation of %CSP.StreamServer which I'm using to download a PDF file, however, any file downloaded is 5Kb smaller than the original. This is the code I'm using:

Class GMECC.DocmanConnect.Pages.downloadGPNotOnHub Extends (%ZEN.Component.page, %CSP.StreamServer)
{
    ClassMethod GetId()
    {
        Quit$Get(%request.Data("filepath",1))
    }

    ClassMethod OnPreHTTP() As%Boolean
    {
        set filepath = %request.Get("filepath")
        
        set tFs=##class(%Stream.FileCharacter).%New()
        set tFs.Filename=filepath
        
        set%response.ContentType = "application/pdf"do%response.SetHeader("Content-Disposition","attachment;filename="""_$P(filepath,"\",*)_"""")
        Do%response.SetHeader("Content-Length",tFs.SizeGet())
    
        Quit$$$OK
    }

    ClassMethod OnPage() As%Status
    {
        set myfile = ##class(%File).%New(%request.Get("filepath"))
        do myfile.Open("S")
        do myfile.OutputToDevice(myfile.SizeGet())	//myfile.OutputToDevice() quit$$$OK
    }
}

And it is called via a hyperlink as follows:

GMECC.DocmanConnect.Pages.downloadGPNotOnHub?filepath=E:\blah\blah.pdf

The resultant file is correctly interpreted as a PDF, given the appropriate file name etc, but is 5Kb smaller

Any thoughts?