If I open a PDF document in Adobe Reader, it usually opens with Fit Width page zoom by default.
But if I click on a bookmark, the zoom changes to Fit Page.
How do I stop this from happening and have Adobe Reader maintain the default zoom?
If you cannot change this behavior by fiddling with Edit/Preference/Page Display/Zoom settings, then you may well have the case of the bookmark being tied to an "Action".
Unfortunately, you cannot change that with Acrobat Reader -- you'd need access to Adobe Acrobat or to Acrobat Professional (or maybe some other tool with similar capabilities in this respect).
Here are some screenshots which illustrate what I mean:
Right-click the bookmark and move mouse pointer to "Properties..." on lower end of context menu...
...and the dialog to edit/add/delete "Actions" appears