pdfium源码包含了pdf的所有操作。这里基于PdfiumAndroid做个扩展，提供合并和分割PDF操作API

PdfWriter

修改后的pdf文档需要保存，首先需要提供写入文件操作

public class PdfWriter {
    private ParcelFileDescriptor mOutput;

    public PdfWriter(ParcelFileDescriptor output) {
        mOutput = output;
    }

    public void write(PdfDocument doc) throws IOException {
        synchronized (PdfiumCore.sPdfiumLock) {
            throwIfClosed();
            nativeWrite(doc.mNativeDocPtr, mOutput.getFd());
        }
    }

    private static native boolean nativeWrite(long docPtr, int fd);

    private void throwIfClosed() {
        if (mOutput == null) {
            throw new IllegalStateException("Already closed");
        }
    }
}

对应JNI实现如下：

//
// Created by svenj on 2019/2/3.
//

extern "C" {
#include <unistd.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <string.h>
#include <stdio.h>
}

#include <fpdfview.h>
#include <fpdf_save.h>
#include "util.hpp"

struct PdfToFdWriter : FPDF_FILEWRITE {
    int dstFd;
};

static bool writeAllBytes(const int fd, const void *buffer, const size_t byteCount) {
    char *writeBuffer = static_cast<char *>(const_cast<void *>(buffer));
    size_t remainingBytes = byteCount;
    while (remainingBytes > 0) {
        ssize_t writtenByteCount = write(fd, writeBuffer, remainingBytes);
        if (writtenByteCount == -1) {
            if (errno == EINTR) {
                continue;
            }
            LOGE("Error writing to buffer: %d", errno);
            return false;
        }
        remainingBytes -= writtenByteCount;
        writeBuffer += writtenByteCount;
    }
    return true;
}

static int writeBlock(FPDF_FILEWRITE* owner, const void* buffer, unsigned long size) {
    const PdfToFdWriter* writer = reinterpret_cast<PdfToFdWriter*>(owner);
    const bool success = writeAllBytes(writer->dstFd, buffer, size);
    if (success < 0) {
        LOGE("Cannot write to file descriptor. Error:%d", errno);
        return 0;
    }
    return 1;
}

JNI_FUNC(jboolean, PdfWriter, nativeWrite)(JNI_ARGS, jlong docPtr, jint fd) {
    auto document = reinterpret_cast<FPDF_DOCUMENT>(docPtr);
    PdfToFdWriter writer;
    writer.dstFd = fd;
    writer.WriteBlock = &writeBlock;
    FPDF_BOOL success = FPDF_SaveAsCopy(document, &writer, FPDF_NO_INCREMENTAL);
    if (!success) {
        jniThrowExceptionFmt(env, "java/io/IOException",
                "cannot write to fd. Error: %d", errno);
    }
}