What are you trying to achieve?
(setq-default pdf-view-display-size 'fit-width)
What have you tried?
Add
(setq-default pdf-view-display-size 'fit-width)
to config.el.
However, this doesn't work as it's overridden by pdf module during use-package! pdf-tools :config.
The canonical way to configure packages is to wrap your configuration in (after! PACKAGE ...) or in use-package blocks, to ensure that your settings overwrite Doom's defaults.
Give this a try:
(after! pdf-view ; or pdf-tools
(setq-default pdf-view-display-size 'fit-width))
(after! pdf-view ...) doesn't work but (after! pdf-tools ...) works. Thank you!