github.com/gen2brain/go-fitz  强大的读取pdf内容的工具包
github.com/pdfcpu/pdfcpu  给pdf添加水印

file, err := fitz.New("out.pdf")
    if err != nil {
        panic(err)
    }
    totalNum := file.NumPage()
    text, _ := file.Text(totalNum - 1)//读取最后一页的内容
    file.Close()
    s := strings.Split(text, "批次")
    x_cnt := len(s) - 1 //最后一页框体数量
    // fmt.Println(text, file.NumPage(), len(s)-1)
    // return
    c1 := "points:10,rot:0,scale:1 abs,offset:-46 180,fillc: #000000"
    c2 := "points:10,rot:0,scale:1 abs,offset:-46 -96.5,fillc: #000000"
    c3 := "points:10,rot:0,scale:1 abs,offset:365 180,fillc: #000000"
    c4 := "points:10,rot:0,scale:1 abs,offset:365 -96.5,fillc: #000000"
    pages := []string{}

    for i := 1; i <= totalNum; i++ {
        pages = append(pages, strconv.Itoa(i))
    }

    pages_4_cnt := pages[0 : len(pages)-1]

    batchNo := "BT20240301001"
    if len(pages_4_cnt) > 0 {
        api.AddTextWatermarksFile("out.pdf", "out.pdf", pages_4_cnt, true, batchNo, c1, nil)
        api.AddTextWatermarksFile("out.pdf", "out.pdf", pages_4_cnt, true, batchNo, c2, nil)
        api.AddTextWatermarksFile("out.pdf", "out.pdf", pages_4_cnt, true, batchNo, c3, nil)
        api.AddTextWatermarksFile("out.pdf", "out.pdf", pages_4_cnt, true, batchNo, c4, nil)
    }
    pages_last := pages[len(pages)-1:]
    switch x_cnt {
    case 1:
        api.AddTextWatermarksFile("out.pdf", "out.pdf", pages_last, true, batchNo, c1, nil)
    case 2:
        api.AddTextWatermarksFile("out.pdf", "out.pdf", pages_last, true, batchNo, c1, nil)
        api.AddTextWatermarksFile("out.pdf", "out.pdf", pages_last, true, batchNo, c2, nil)
    case 3:
        api.AddTextWatermarksFile("out.pdf", "out.pdf", pages_last, true, batchNo, c1, nil)
        api.AddTextWatermarksFile("out.pdf", "out.pdf", pages_last, true, batchNo, c2, nil)
        api.AddTextWatermarksFile("out.pdf", "out.pdf", pages_last, true, batchNo, c3, nil)
    case 4:
        api.AddTextWatermarksFile("out.pdf", "out.pdf", pages_last, true, batchNo, c1, nil)
        api.AddTextWatermarksFile("out.pdf", "out.pdf", pages_last, true, batchNo, c2, nil)
        api.AddTextWatermarksFile("out.pdf", "out.pdf", pages_last, true, batchNo, c3, nil)
        api.AddTextWatermarksFile("out.pdf", "out.pdf", pages_last, true, batchNo, c4, nil)
    }