Try:

DECLARE LONG VeryCreate IN "verywrite.dll" STRING @ lpFileName
DECLARE VeryClose IN "verywrite.dll" (LONG VERY_ID)
DECLARE LONG VeryAddImage IN "verywrite.dll" (LONG VERY_ID ,STRING @ lpFileName)

I assume that lpFileName is a long pointer to the memory adress of file name string so try to pass it by reference:

FileName = 'test2.pdf'
pdfId = verycreate(FileName)
?pdfId

>VFP unsuccessful code
>

>DECLARE LONG VeryCreate IN "verywrite.dll" (STRING lpFileName)
>DECLARE VeryClose IN "verywrite.dll" (LONG VERY_ID)
>DECLARE LONG VeryAddImage IN "verywrite.dll" (LONG VERY_ID ,STRING lpFileName)
>
>** returns an error message
>** TOO MANY ARGUMENTS
>?  verycreate("test2.pdf")
>

>
>VB successful code (from VB6)
>

>Private Declare Function VeryCreate Lib "verywrite.dll"
>(ByVal lpFileName As String) As Long
>Private Declare Sub VeryClose Lib "verywrite.dll" (ByVal id As Long)
>Private Declare Function VeryAddImage Lib "verywrite.dll"
>(ByVal id As Long, ByVal lpFileName As String) As Long
>
>Private Sub Command1_Click()
>Dim filename As String
>Dim id As Long
>
>filename = "create.pdf"
>id = VeryCreate(filename)
>filename = "multipage.tif"
>ret = VeryAddImage(id,filename)
>...
>

>
>Any hint or trick is welcome. If VB can, then VFP should...
>
>Francis