Would like to know is this happening with other files or with any certain files.
Please check if is there any update available for the application by going through 'Help>Check for Updates' & also provide the Exact OS & application version.
Could please share your workflow how you are adding them.
If possible could you share any sample file so that can replicate the issue at our end.
In response to your questions.
1. Windows 10 Professional 64 bit running Adobe Acrobat DC version 15.006.30174. There are no updates available.
2. So far it seems to be only these four files, which I decline to provide a copy of because they are the federal and state tax returns for four trusts.
3. Workflow after which the problem was seen: Files received from accountant were saved with an alternate name, edited using the edit tool, and organized to delete a few pages of administrative matters, e.g. transmittal letter and invoice pages. Bookmarks for the deleted pages were also removed. After the Bookmarks were recreated by going to the desired section start and right clicking the "Add Bookmark" command, the documents were saved and closed. When next opened, the bookmarks had again disappeared.
4. I have now saved the original files with an alternate name and again edited them, being slightly more careful about what/how exactly I deleted. The bookmarks have not disappeared.
5. I went into properties and checked the metadata. The PDF Properties Section showed that the files were originally created by Amyuni PDF Converter version 220.127.116.11. I now believe that the problem was caused by some glitch generated by this third party software. Acrobat also doesn't recognize the watermark created using the third party software.
I consider this question closed from Adobe's perspective.