If I buy an official campaign like "Out of the Abyss," what can I do to customize it?

[Fudly]

Can I attack the story with a highlighter (or I guess underlines)?
Can I adjust encounter difficulties by adding/removing mobs?
Can I adjust loot parcels by adding/removing loot?
Can I add new pins to the maps?

[LordEntrails]

First, you can't change the module itself. BUT, you can make changes to the entries and such once you have added it to a campaign. So, first you load the module in a campaign, then you can modify the entries in that campaign, (note that the changes will only be for the one campaign, not for all campaigns you use the module in.)

1) Highlighting or Underlining: You can unlock and modify the entries, changing the text or even the style, you can bold, but I don't think their is an underline option (maybe I'm wrong though).
2) Yes you can change the number of NPCs in an encounter, add new NPCs, remove what you want, you can also modify any NPC effectively making a new NPC.
3) Yes
4) Yes, and add new links to story entries, add new story entries, etc

[Fudly]

Nice. For some reason, I expected everything to be locked.

[LordEntrails]

Once its in the campaign, you can click on the unlock icon, then you make modifications that overlay the module to the campaign file.

[Moon Wizard]

Our general approach to modules is that adventure modules can be edited, but the reference modules are marked as read only.

Regards,
JPG

[Nickademus]

If it makes it easier to understand, each campaign has a file that lists changes made to each module. You don't actually change anything, but FG reads the data of the module, then reads the file to see if that data has been changed. The module remains untouched and you will have to change something in every campaign you open the module in (so that campaign adds the alteration to the list in the file).