Ignore .pdf

Ignore .pdf

On Windows, when PDF is opened with Acrobat Reader, one gets following output:

    ! I can't write on file `document.pdf'.
    Please type another file name for output:

If one simply presses "Enter" to continue, the file `.pdf` is generated. Since this is a common case and `.pdf` is never used as full file name, this file should be ignored, too.