I want to include Microblink's PDF417 framework into my library. Library project compile and work fine but when I use MyLibrary.a file in my application I've got "undefined symbols for architecture armv7" error. Any ideas? Can I include custom framework to library or this isn't possible.
Trojanfoe's answer is correct for your case. But in general, the answer depends on the type of the library inside the framework.
iOS/MacOS framework is a just a collection of a library together with all relevant header files. This makes including the library into other projects much easier, because the whole framework can be included at once, thus eliminating the need to modify linker and header search paths and linker flags.
Library itself can be either a static library or a dynamic/shared library. Framework can contain the library of any type, there are no limitations in that regard.
If the library in framework is static, then all the objects from that library are copied into target product at compile time. If the target product is a static library (MyLibrary.a in your case), additional linking with the framework in the application is not needed, because all the objects are contained in MyLibrary.a
If the library in the framework is dynamic, then objects from that library are loaded at load-time or run-time, not at compile time. Because of that, frameworks of that type need to be linked with end applications also.
In your case, pdf417 framework contains a dynamic library, which means you will also have to include that framework into your end application.
I'm a developer on Microblink's PDF417 SDK. The thing is, we can provide our library in any format. The format we have chosen in our Github repository is an .embeddedframework which contains a dynamic library together with all resource files because that makes including the framework into Application projects very simple. If you have a use case which requires a different format, we invite you to contact us on https://help.microblink.com/hc/en-us