I found the general help from Android documentation to be helpful:
https://developer.android.com/studio/projects/add-native-code

Since PDFWriter is a Cmake project it can be easily incorporated into a new or existing application.
there's several ways to go about this, but i find the simplest is to add a new native C++ library to your project, which will involve having a new CMakeList.txt file and some native code. In that CMakeLists.txt refer to pdf-writer, and then write code in the native library to do whatever you want with pdf-writer.

So for example if one uses the "Native C++" project template you can add the following steps and have pdf-writer available for you:

1. Edit the CMakeLists.txt to fetch PDF-Writer from github

In the CMakeLists.txt file of the native-lib add the following statement to fetch pdf-writer:

# hummus dependency
include(FetchContent)
FetchContent_Declare(
        PDFHummus
        GIT_REPOSITORY https://github.com/galkahana/PDF-Writer.git
        GIT_TAG        v4.5.7
)
FetchContent_MakeAvailable(PDFHummus)

2. Add pdf-writer dependency to your native library

Still in the CMakeLists.txt file of the native-lib add this statement to link pdf-writer to your library and make includes and links available:

target_link_libraries (mynativeapplication PDFHummus::PDFWriter)

(change mynativeapplication to the name used in add_library).

3. Add code in the exported function(s) to use PDF-Writer

for example, in the native app template i added the code in the native-lib.cpp.

here's the full code example, it creates an empty file:

#include <jni.h>
#include <string>

#include "PDFWriter.h"
#include "PDFPage.h"
#include "PDFRectangle.h"

#include <iostream>

using namespace std;
using namespace PDFHummus;

extern "C" JNIEXPORT jstring JNICALL
Java_com_example_mynativeapplication_MainActivity_stringFromJNI(
        JNIEnv* env,
        jobject /* this */) {

    // build me an empty PDF
    PDFWriter pdfWriter;
    EStatusCode status;

    do
    {
        status = pdfWriter.StartPDF("./EmptyPages.pdf",ePDFVersion14);
        if(status != PDFHummus::eSuccess)
        {
            cout<<"failed to start PDF\n";
            break;
        }


        PDFPage* page = new PDFPage();
        page->SetMediaBox(PDFRectangle(0,0,595,842));

        for(int i=0;i<4 && PDFHummus::eSuccess == status;++i)
        {
            status = pdfWriter.WritePage(page);
            if(status != PDFHummus::eSuccess)
                cout<<"failed to write page "<<i<<"\n";
        }
        delete page;

        status = pdfWriter.EndPDF();
        if(status != PDFHummus::eSuccess)
        {
            cout<<"failed in end PDF\n";
            break;
        }
    }while(false);



    std::string hello = "Hello from C++";
    return env->NewStringUTF(hello.c_str());
}

This should create a file...somewhere (i guess pointing to a clear place in the device would be better...but it's just an example, so i allowed it).
I'm guessing similar thing can be done for other usages.

Having said that

I'm kinda positive there are Java PDF libraries, no? nothing that can be used instead that might make this simpler? though i guess it doesn't look very complicated. I hear PDFBox is quite good, if you are interested in looking for an alternative.

from pdf-writer.

Can be helpful but noy exactly because my goal is developing a crossplatform Unreal Engine plugin. So, there is no Android Studio project at all. I need a gradlew assembleRelease like solution to give me .so files

from pdf-writer.

I see. I think this is covered by adding and externalNativeBuild statement to your gradle:
https://developer.android.com/studio/projects/gradle-external-native-builds#configure-gradle

The studio just makes it easy in creating a local cmake project for you that you can tie to pdf-,writer. The end result is an so library.

But yeah im no android/gradle expert so maybe it doesn't help

from pdf-writer.