Document Viewer is part of the 11.04 release, yet it does not appear in the dash anywhere. It is installed, but it cannot be found by a search when in the dash and it does not appear as an installed application. What other preinstalled applications are missing from the dash and why was the dash made so difficult to customize?
I don't know if my answer will be really helpful, but the Document-Viewer (Evince in fact) is also the default program for viewing PDF files on 10.10 and it is not possible to find it in the OS-menu, and a launcher like Synapse also has a hard time finding it (it's deep in the search list). Another program behaving this way is the File Roller (used to compress and decompress files).
I believe the idea is that those programs are not used actively, but passively. By this i mean that you do not open the file viewer and then chose the file you want to view, create a new file from there. Similarly you do not open the file roller and then choose a file to compress or decompress. On the contrary users will open their files and the file's mimetype will tell what program should be associated with it. A PDF opens Evince and a TAR.GZ opens File Roller.
Since users don't really need to launch those programs, they are not added to the OS menu or dash in order to save space and gain visibility and access.
As to why the dash is so difficult to customize, i have no answer. It may depend on how much you feel you need to customize. If what you really want is customization, perhaps the Unity interface is not what you need. Programs like Synapse and GnomeDo will do the search and Cairo-Dock or Docky can be used as docks.