How are you loading the document? If it's not a file on disk that you pass directly via documentViewer.Document = "myfile.pdf", i.e. you are loading it from a stream or byte array, then you are responsible for providing a uniqueID that varies when your file is changed. The point of uniqueID is to identify a binary version of document.
- Type: System.String
The unique identifier that will be used for generating the cache key for this document. For instance, it can be an ID from your database table or a simple file name; you just need to make sure this ID varies for each different document so that they are cached correctly. For example for files on disk, we internally use a string combination of file extension, file size and file date for uniquely identifying them, this way cache collisions do not occur and we can resuse the cached file even if the file name before extension is changed (because it's still the same document).
So you need to vary your uniqueID, otherwise same cache key will be generated and already cached version will be loaded. As you would notice uniqueID is a string so you can vary it as you would like, e.g. append a version number or DateTime.Now.Ticks.