I will carefully define a bookmark that coincides with sections of my document. The bookmarks contain a range of pages from part of one to many. At some undermined time or following some undetermined action, the bookmarks often move downward (toward the end of the document) by a few lines of text. I have not been able to isolate this phenomenon. Someone suggested that it might be due to my styles, which were not carefully done.

The start/end positions of a bookmark are liable to expand if you press the enter key anywhere within the bookmarked range, including at the start or end of that range. A bookmark's range will likewise contract if you delete content within that range.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]

I realize that an insertion inside a bookmark will expand the bookmark to match, and deleting something inside a bookmark will make it smaller. That is exactly what I expect. But I am seeing both ends of a bookmark moving down, for no apparent reason, by several lines toward the end of the document. I don't see what I could do inside the bookmark to cause this. I also find it strange that apparently no one else has reported this problem. I am using Word 365-2016 in compatibility mode on Win 7. I have not been able to isolate what actions I am doing that cause this: changing styles (which are pretty disorganized), adding text elsewhere, saving and opening the document, etc.

I've used Word for around 25 years and have never seen such behaviour, so I can't suggest a design reason for it. Changing Styles & editing other portions of the document are unlikely to be contributing factors. More likely it's document corruption or a faulty Office installation.

Corrupt documents can often be 'repaired' by inserting a new, empty, paragraph at the very end, copying everything except that new paragraph to a new document based on the same template (headers & footers may need to be copied separately), closing the old document and saving the new one over it.

Similarly, in case it's a problem with the Office installation, you could try repairing it (via Start > Windows Control Panel > Programs > Programs & Features > Microsoft Office (version) > Change).

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]