pdf creation has been disabled

[dpshelio]

pdf was disabled on #17

Should we enabled it back? has been used? I remember to see students year ago that downloaded it... but not sure whether they used it.
The error that happens is:

...
Chapter 3.

Underfull \hbox (badness 10000) in paragraph at lines 938--940
[]\TU/lmr/bx/n/10 Supplementary materials\TU/lmr/m/n/10 : The escaping rules ar
e quite subtle. See
[10] [11] [12] [13] [14]
! Unable to load picture or PDF file 'combined_files/combined_154_0.png'.
<to be read again> 
                   }
l.1339 ...ght}}{combined_files/combined_154_0.png}
                                                  
? 
! Emergency stop.
<to be read again> 
                   }
l.1339 ...ght}}{combined_files/combined_154_0.png}
                                                  
Output written on notebook.pdf (15 pages).
Transcript written on notebook.log.

make: *** [Makefile:24: notes.pdf] Error 1
Error: Process completed with exit code 2.

Maybe the pictures are not generated as external files any more.