用C++代码实现为PDF文件添加文本水印类型的注释

可以使用libharu库来实现为PDF文件添加文本水印类型的注释。以下是一个简单的示例代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <time.h>
#include <stdarg.h>
#include <ctype.h>
#include <setjmp.h>
#include <assert.h>
#include "hpdf.h"

int main(int argc, char **argv)
{
    HPDF_Doc  pdf;
    HPDF_Page page;
    HPDF_Font font;
    HPDF_REAL width, height;
    HPDF_Point pos;
    const char *text = "Watermark Text";
    const char *filename = "output.pdf";
    const char *owner_password = NULL;
    const char *user_password = NULL;
    HPDF_EncryptMode enc_mode = HPDF_ENCRYPT_R2;
    HPDF_STATUS status;

    pdf = HPDF_New(NULL, NULL);
    if (!pdf) {
        printf("error: cannot create PDF object\n");
        return 1;
    }

    /* set encryption mode */
    status = HPDF_SetEncryptionMode(pdf, enc_mode, 128);
    if (status != HPDF_OK) {
        printf("error: cannot set encryption mode\n");
        HPDF_Free(pdf);
        return 1;
    }

    /* set passwords */
    status = HPDF_SetPassword(pdf, owner_password, user_password);
    if (status != HPDF_OK) {
        printf("error: cannot set passwords\n");
        HPDF_Free(pdf);
        return 1;
    }

    /* add a new page */
    page = HPDF_AddPage(pdf);
    if (!page) {
        printf("error: cannot add new page\n");
        HPDF_Free(pdf);
        return 1;
    }

    /* set font and font size */
    font = HPDF_GetFont(pdf, "Helvetica", NULL);
    HPDF_Page_SetFontAndSize(page, font, 24);

    /* get text width and height */
    width = HPDF_Page_TextWidth(page, text);
    height = HPDF_Page_GetHeight(page);

    /* set text position */
    pos.x = (HPDF_Page_GetWidth(page) - width) / 2;
    pos.y = (height - 24) / 2;

    /* set text color */
    HPDF_Page_SetRGBFill(page, 0.5, 0.5, 0.5);

    /* draw text */
    HPDF_Page_BeginText(page);
    HPDF_Page_TextOut(page, pos.x, pos.y, text);
    HPDF_Page_EndText(page);

    /* save the document to a file */
    status = HPDF_SaveToFile(pdf, filename);
    if (status != HPDF_OK) {
        printf("error: cannot save PDF file\n");
        HPDF_Free(pdf);
        return 1;
    }

    /* clean up */
    HPDF_Free(pdf);

    return 0;
}

这个示例代码使用libharu库创建了一个PDF文档，并在其中添加了一个文本水印。你可以根据自己的需求修改文本内容、字体、字号、颜色、位置等参数。