pdf.js移动端预览PDF文件时，支持双指缩放

在viewer.html中添加手势缩放代码

<script>
    // alert("Hello World");
    let agent = navigator.userAgent.toLowerCase();
    // if (!agent.includes("iphone")) {
    let pinchZoomEnabled = false;

    function enablePinchZoom(pdfViewer) {
      let startX = 0, startY = 0;
      let initialPinchDistance = 0;
      let pinchScale = 1;
      const viewer = document.getElementById("viewer");
      const container = document.getElementById("viewerContainer");

      const reset = () => {
        startX = startY = initialPinchDistance = 0;
        pinchScale = 1;
      };

      // Prevent native iOS page zoom
      // document.addEventListener("touchmove", (e) => { if (e.scale !== 1) { e.preventDefault(); } }, { passive: false });

      document.addEventListener("touchstart", (e) => {
        if (e.touches.length > 1) {
          startX = (e.touches[0].pageX + e.touches[1].pageX) / 2;
          startY = (e.touches[0].pageY + e.touches[1].pageY) / 2;
          initialPinchDistance = Math.hypot(
            e.touches[1].pageX - e.touches[0].pageX,
            e.touches[1].pageY - e.touches[0].pageY
          );
        } else {
          initialPinchDistance = 0;
        }
      });

      document.addEventListener(
        "touchmove",
        (e) => {
          if (initialPinchDistance <= 0 || e.touches.length < 2) {
            return;
          }
          if (e.scale !== 1) {
            e.preventDefault();
          }
          const pinchDistance = Math.hypot(
            e.touches[1].pageX - e.touches[0].pageX,
            e.touches[1].pageY - e.touches[0].pageY
          );
          const originX = startX + container.scrollLeft;
          const originY = startY + container.scrollTop;
          pinchScale = pinchDistance / initialPinchDistance;
          viewer.style.transform = `scale(${pinchScale})`;
          viewer.style.transformOrigin = `${originX}px ${originY}px`;
        },
        { passive: false }
      );

      document.addEventListener("touchend", (e) => {
        if (initialPinchDistance <= 0) {
          return;
        }
        viewer.style.transform = `none`;
        viewer.style.transformOrigin = `unset`;
        PDFViewerApplication.pdfViewer.currentScale *= pinchScale;
        const rect = container.getBoundingClientRect();
        const dx = startX - rect.left;
        const dy = startY - rect.top;
        container.scrollLeft += dx * (pinchScale - 1);
        container.scrollTop += dy * (pinchScale - 1);
        reset();
      });
    }

    document.addEventListener("DOMContentLoaded", () => {
      if (!pinchZoomEnabled) {
        pinchZoomEnabled = true;
        enablePinchZoom();
      }
    });
  </script>