I've just found the quote inside the Emacs manual, but further, I realized now that in the DocView documentation I had seen, between the

C-c Prefix Command

line (above) and the

+ doc-view-enlarge

line (below) there is another Prefic Command line

ESC Prefix Command

so it was my fail typing C-c before +, which was the reason for no working. If I do ESC +, it works fine.

Sorry for the spam. Thanks!

Iago

Iago <iago.gine@sjd.es> writes:

I started Emacs with emacs -Q. Then, open any pdf file and try to use zoom 
capabilities of DocView mode. Reading the DocView documentation I 
understand that to doc-view-enlarge, first I should type C-c (as the 
Prefix Command) followed by typing +. But this produces the message 
C-c + is undefined 

Where do you read this?  In Emacs 27.1 in the Emacs manual, it says:

---
  You can enlarge or shrink the document with @kbd{+}
(@code{doc-view-enlarge}) and @kbd{-} (@code{doc-view-shrink}).
---

--
Iago Giné-Vázquez (0000-0002-6725-2638)

Centro de Investigación Biomédica en Red de Salud Mental, Instituto de Salud Carlos III (CIBERSAM, ISCIII), Av. Monforte de Lemos, 3-5. Pabellón 11. Planta 0, 28029 Madrid, Spain
Epidemiology of Mental Health Disorders and Ageing, Institut de Recerca Sant Joan de Déu, Santa Rosa 39-57, 08950 Esplugues de Llobregat, Spain
Research, Innovation and Teaching Unit, Parc Sanitari Sant Joan de Déu, Doctor Antoni Pujadas 42, 08830 Sant Boi de Llobregat, Spain