Hello @Cheryl7434 ,
I'm having trouble recreating the issue you're describing. In the CSCTEST copy I made in your account, no matter how I chose to replace a document button or its link, I always got the expected PDF / Word document or link redirect.
What exactly is the process you're going through when replacing a document link in your emails? Are you actually removing the link from the button before trying to add a new link to it? Or are you completely deleting the linked button / text / image and inserting a new linked item?
Are you saving your work, as well as doing a preview or test send to check the new links, prior to sending out live to your contacts?
It may be worth calling into our regular support line so they can do a screenshare and see the issue you're describing occur with you live, as it's happening.