iOS error: Undefined symbols for architecture i386 (or armv7, armv7s, arm64, x86_64)

Q: I am receiving the following error message when I try to use libPDFNet.a
in my project. It doesn't make any sense because I have used lipo to
confirm that the library does have an i386 slice. Can you tell me what's
going on??

ld: warning: ignoring file /Users/PDFTron/PDFNet_6.1/iOS/Sample/Lib/libPDFNet.a,
missing required architecture i386 in file /Users/PDFTron/PDFNet_6.1/iOS/Sample/Lib/libPDFNet.a
(1 slices)

Undefined symbols for architecture i386:

  "_OBJC_CLASS_$_PDFDoc", referenced from:

      objc-class-ref in RootViewController.o

  "_OBJC_CLASS_$_PanTool", referenced from:

      objc-class-ref in RootViewController.o

  "_OBJC_CLASS_$_PDFViewCtrl", referenced from:

      objc-class-ref in RootViewController.o

     (maybe you meant: _OBJC_CLASS_$_PDFViewCtrlAppDelegate)

  "_OBJC_CLASS_$_Print", referenced from:

      objc-class-ref in RootViewController.o

  "_OBJC_CLASS_$_PDFNet", referenced from:

      objc-class-ref in RootViewController.o

ld: symbol(s) not found for architecture i386

clang: error: linker command failed with exit code 1 (use -v to see
invocation)
-------------------------------

A:

You may get this error message if you are using a version of libPDFNet.a
that was compiled with a newer version of LLVM than you are using to
compile your project. Please make sure XCode is up to date.