[gricoo2017]

Hi guys,
I admit I am a noob, please be gentle.
I am using lastest calibre on a ubuntu 17.10. machine.
My problem is I can not open e-books with pdf extension. epub&mobi etc. is no problem.

I am getting following Error:
Traceback (most recent call last):
File "site-packages/calibre/utils/ipc/simple_worker.py", line 284, in main
File "site-packages/calibre/ebooks/oeb/iterator/book.py", line 54, in extract_book
File "site-packages/calibre/ebooks/conversion/plumber.py", line 720, in __init__
ValueError: Input file must have an extension

Any guidance most appreciated.
Cheers grico