Is it possible to spell check PDF files

Turns out I figured it out not long after typing this question! I just thought I'd experiment a little and found out how to do it.

You change the language in Edit > Spelling > Spelling... and then reopen the document. That did it for me. I'm guessing it is a bug: I don't think having to close and open a document for language error display settings to make effect it's very intuitive, but I'm glad it can be done!

I'm posting this anyways to share the knowledge and in the hopes it will be useful to somebody else, struggling with those dotted red lines!

When you press the Learn button, the word is added to ~/Library/Spelling/LocalDictionary if the language is set to automatic or to ~/Library/Spelling/en if the language is set to U.S. English. en_GB is used for British English and en is used only for U.S. English.

If you edit the files in ~/Library/Spelling/ directly, you can apply the changes by terminating the AppleSpell process.

So try to assign a shortcut to a script like this:

cd ~/Library/Spelling
if [[ -e fiction ]]; then
  mv en normal; mv fiction en; d=fiction
else
  mv en fiction; mv normal en; d=normal
fi
killall AppleSpell
terminal-notifier -title "Using dictionary $d" -message '' -group spellingdictionary
sleep 3
terminal-notifier -remove spellingdictionary

If for example you use the British English dictionary, change en to en_GB.

You can install terminal-notifier with sudo /usr/bin/gem install terminal-notifier.