Android : Getting uknown error when using mupdf

I've tried to import mupdf to my app, but when I open it, i got some weird error and it force closes. Why? I really don't understand.

this is my logcat:

10-10 15:34:17.802    8657-8657/com.tproductions.Openit E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.tproductions.Openit, PID: 8657
java.lang.UnsatisfiedLinkError: Native method not found: com.artifex.mupdfdemo.MuPDFCore.openFile:(Ljava/lang/String;)J
        at com.artifex.mupdfdemo.MuPDFCore.openFile(Native Method)
        at com.artifex.mupdfdemo.MuPDFCore.<init>(MuPDFCore.java:68)
        at com.artifex.mupdfdemo.MuPDFActivity.openFile(MuPDFActivity.java:204)
        at com.artifex.mupdfdemo.MuPDFActivity.onCreate(MuPDFActivity.java:303)
        at android.app.Activity.performCreate(Activity.java:5231)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2201)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2286)
        at android.app.ActivityThread.access$800(ActivityThread.java:144)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1246)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:212)
        at android.app.ActivityThread.main(ActivityThread.java:5135)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:877)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:693)
        at dalvik.system.NativeStart.main(Native Method)