The code you've posted already allows upload of DOC files.
To allow PDF upload, change the following line:
case ".jpg", ".jpeg", ".gif", ".zip", ".doc", ".txt"
to
case ".jpg", ".jpeg", ".gif", ".zip", ".doc", ".txt", ".pdf"
You can set the allowed maximum size of a file in the following line:
If allowFile and byteCount <= 250000 Then
Simply replace the blue value to your desired maximum size in bytes.