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.
# *.eps
# *.pdf
+## Generated if empty string is given at "Please type another file name for output:"
+.pdf
+
## Bibliography auxiliary files (bibtex/biblatex/biber):
*.bbl
*.bcf