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)

PUBLIC iVeryHandle
ERASE mytest.pdf

** below OK
iVeryHandle = VeryCreate("mytest.pdf")
? "My handle on the stuff:",iVeryHandle

lcImageName = "mytest.tif"+chr(0)
? VeryAddImage(iVeryHandle,lcImageName)
? VeryClose(iVeryHandle)

I remeber the I had troubles with some API. Worked well only if the passed string was null terminated
lcImageName = 'mytest.pdf'+chr(0)

Try this way