A solution other than a specific PDF reader is to use an external program such as GoldenDict. It features a so-called ”Scan Popup“: Each time you select a word in any program—e.g. in your favourite PDF reader—a popup containing the lookup result will show up.
(*) trom the GoldenDict in-application help: [source code]
To look up words from other active applications, you would need to first activate the "Scan popup functionality" in Preferences, and then enable it at any time either by triggering the '[Scan] Popup' icon [in the toolbar], or by clicking the tray icon [..] with your right mouse button and choosing so in the menu you've popped.