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2v5a1 1 0 0 0 1 1h5M10 9H8m8 4H8m8 4H8\"\u002F>\u003C\u002Fg>",{"left":4,"top":4,"width":5,"height":5,"rotate":4,"vFlip":6,"hFlip":6,"body":33},"\u003Cg fill=\"none\" stroke=\"currentColor\" stroke-linecap=\"round\" stroke-linejoin=\"round\" stroke-width=\"2\">\u003Cpath d=\"M21 8a2 2 0 0 0-1-1.73l-7-4a2 2 0 0 0-2 0l-7 4A2 2 0 0 0 3 8v8a2 2 0 0 0 1 1.73l7 4a2 2 0 0 0 2 0l7-4A2 2 0 0 0 21 16Z\"\u002F>\u003Cpath d=\"m3.3 7l8.7 5l8.7-5M12 22V12\"\u002F>\u003C\u002Fg>",{"left":4,"top":4,"width":5,"height":5,"rotate":4,"vFlip":6,"hFlip":6,"body":27},{"left":4,"top":4,"width":5,"height":5,"rotate":4,"vFlip":6,"hFlip":6,"body":36},"\u003Cg fill=\"none\" stroke=\"currentColor\" stroke-linecap=\"round\" stroke-linejoin=\"round\" stroke-width=\"2\">\u003Crect width=\"18\" height=\"18\" x=\"3\" y=\"3\" rx=\"2\" ry=\"2\"\u002F>\u003Ccircle cx=\"9\" cy=\"9\" r=\"2\"\u002F>\u003Cpath d=\"m21 15l-3.086-3.086a2 2 0 0 0-2.828 0L6 21\"\u002F>\u003C\u002Fg>",{"left":4,"top":4,"width":5,"height":5,"rotate":4,"vFlip":6,"hFlip":6,"body":38},"\u003Cg fill=\"none\" stroke=\"currentColor\" stroke-linecap=\"round\" stroke-linejoin=\"round\" stroke-width=\"2\">\u003Crect width=\"18\" height=\"18\" x=\"3\" y=\"3\" rx=\"2\"\u002F>\u003Cpath d=\"M16 8.9V7H8l4 5l-4 5h8v-1.9\"\u002F>\u003C\u002Fg>",{"left":4,"top":4,"width":5,"height":5,"rotate":4,"vFlip":6,"hFlip":6,"body":40},"\u003Cpath fill=\"none\" stroke=\"currentColor\" stroke-linecap=\"round\" stroke-linejoin=\"round\" stroke-width=\"2\" d=\"M6 11h4M8 9v4m7-1h.01M18 10h.01m-.69-5H6.68a4 4 0 0 0-3.978 3.59l-.017.152C2.604 9.416 2 14.456 2 16a3 3 0 0 0 3 3c1 0 1.5-.5 2-1l1.414-1.414A2 2 0 0 1 9.828 16h4.344a2 2 0 0 1 1.414.586L17 18c.5.5 1 1 2 1a3 3 0 0 0 3-3c0-1.545-.604-6.584-.685-7.258q-.01-.075-.017-.151A4 4 0 0 0 17.32 5\"\u002F>",{"left":4,"top":4,"width":5,"height":5,"rotate":4,"vFlip":6,"hFlip":6,"body":42},"\u003Cg fill=\"none\" stroke=\"currentColor\" stroke-linecap=\"round\" stroke-linejoin=\"round\" stroke-width=\"2\">\u003Cpath d=\"M11.017 2.814a1 1 0 0 1 1.966 0l1.051 5.558a2 2 0 0 0 1.594 1.594l5.558 1.051a1 1 0 0 1 0 1.966l-5.558 1.051a2 2 0 0 0-1.594 1.594l-1.051 5.558a1 1 0 0 1-1.966 0l-1.051-5.558a2 2 0 0 0-1.594-1.594l-5.558-1.051a1 1 0 0 1 0-1.966l5.558-1.051a2 2 0 0 0 1.594-1.594zM20 2v4m2-2h-4\"\u002F>\u003Ccircle cx=\"4\" cy=\"20\" r=\"2\"\u002F>\u003C\u002Fg>",{"left":4,"top":4,"width":5,"height":5,"rotate":4,"vFlip":6,"hFlip":6,"body":44},"\u003Cg fill=\"none\" stroke=\"currentColor\" stroke-linecap=\"round\" stroke-linejoin=\"round\" stroke-width=\"2\">\u003Cpath d=\"M21.42 10.922a1 1 0 0 0-.019-1.838L12.83 5.18a2 2 0 0 0-1.66 0L2.6 9.08a1 1 0 0 0 0 1.832l8.57 3.908a2 2 0 0 0 1.66 0zM22 10v6\"\u002F>\u003Cpath d=\"M6 12.5V16a6 3 0 0 0 12 0v-3.5\"\u002F>\u003C\u002Fg>","\u003Cblockquote>\n\u003Cp>一份扫描合同三十多 MB，邮箱限制 20MB、微信发文件也卡，重新扫又麻烦。问题的根子不是"页数多"，而是扫描 PDF 本质上是一叠高分辨率图片。看懂这点，就知道该压哪里、能压到多小、什么时候压不动。\u003C\u002Fp>\n\u003C\u002Fblockquote>\n\u003Cp>\u003Cimg src=\"\u002Fblog\u002Fcompress-scanned-pdf\u002Fcover.webp\" alt=\"扫描 PDF 是一叠高分辨率位图，压缩后变小\">\u003C\u002Fp>\n\u003Ch2>扫描的 PDF 为什么这么大？\u003C\u002Fh2>\n\u003Cp>因为扫描件 PDF 里存的不是文字，而是\u003Cstrong>每页一张位图照片\u003C\u002Fstrong>。扫描仪或手机把纸拍成图片，再塞进 PDF 容器，所以决定体积的是图片的分辨率和色深，跟这页有多少字毫无关系——一页空白纸和一页密密麻麻的合同，扫出来的大小可能差不多。\u003C\u002Fp>\n\u003Cp>算一笔账就清楚了：一页 A4 用 300 DPI 彩色扫描，约 2480×3508 像素，未压缩的位图接近 25MB。哪怕扫描软件做了基础 JPEG 压缩，单页也常有 1–3MB，十几页叠起来轻松几十 MB。这和从 Word 直接"导出 PDF"的文字型文档是两回事：后者存的是文字和矢量，一份几十页的报告可能才几百 KB。\u003C\u002Fp>\n\u003Cp>所以判断一份 PDF 能不能压、能压多少，第一步是分清它是\u003Cstrong>文字型\u003C\u002Fstrong>还是\u003Cstrong>图片\u002F扫描型\u003C\u002Fstrong>。前者压缩空间小，后者才是压缩的主战场。\u003C\u002Fp>\n\u003Ch2>压缩 PDF 到底在压什么？\u003C\u002Fh2>\n\u003Cp>PDF 压缩主要做两件事：\u003Cstrong>去掉冗余结构\u003C\u002Fstrong>和\u003Cstrong>重新编码图片\u003C\u002Fstrong>。对扫描件来说，真正省体积的是后者。\u003C\u002Fp>\n\u003Cul>\n\u003Cli>\u003Cstrong>无损优化\u003C\u002Fstrong>：清理重复对象、未使用的资源、冗余的元数据，把内部结构整理紧凑。这一步不改任何画面，但对纯图片 PDF 省不了多少。\u003C\u002Fli>\n\u003Cli>\u003Cstrong>图片重编码\u003C\u002Fstrong>：把页面里的位图按目标分辨率重新采样，再用 JPEG 之类的有损算法压缩。这是扫描件能从几十 MB 掉到几 MB 的关键——降一半分辨率，数据量大约降到四分之一。\u003C\u002Fli>\n\u003C\u002Ful>\n\u003Cp>一个稳妥的做法是\u003Cstrong>两阶段压缩\u003C\u002Fstrong>：先做无损整理，再对每一页图片逐页重编码。这样既不会无脑破坏画质，又能在图片层拿到主要收益。\u003Ca href=\"\u002Fdocs\u002FpdfCompress\u002F\">PDF 压缩工具\u003C\u002Fa>就是按这个思路，对扫描件和图片型 PDF 通常能压到原大小的 10%–30%。\u003C\u002Fp>\n\u003Ch2>压到多小才不糊？分辨率怎么定\u003C\u002Fh2>\n\u003Cp>关键结论先说：\u003Cstrong>屏幕阅读和普通打印，150–200 DPI 就够用\u003C\u002Fstrong>，再高基本是浪费；低于 100 DPI 才会明显发虚。用途决定该保留多少分辨率：\u003C\u002Fp>\n\u003Ctable>\n\u003Cthead>\n\u003Ctr>\n\u003Cth>用途\u003C\u002Fth>\n\u003Cth>建议分辨率\u003C\u002Fth>\n\u003Cth>说明\u003C\u002Fth>\n\u003C\u002Ftr>\n\u003C\u002Fthead>\n\u003Ctbody>\n\u003Ctr>\n\u003Ctd>微信\u002F邮件传阅、屏幕看\u003C\u002Ftd>\n\u003Ctd>120–150 DPI\u003C\u002Ftd>\n\u003Ctd>体积最小,屏幕上完全够清晰\u003C\u002Ftd>\n\u003C\u002Ftr>\n\u003Ctr>\n\u003Ctd>普通打印、存档\u003C\u002Ftd>\n\u003Ctd>200–300 DPI\u003C\u002Ftd>\n\u003Ctd>打印不发虚,体积适中\u003C\u002Ftd>\n\u003C\u002Ftr>\n\u003Ctr>\n\u003Ctd>需要 OCR 或放大看细节\u003C\u002Ftd>\n\u003Ctd>≥300 DPI\u003C\u002Ftd>\n\u003Ctd>不建议大幅压缩,优先保真\u003C\u002Ftd>\n\u003C\u002Ftr>\n\u003C\u002Ftbody>\n\u003C\u002Ftable>\n\u003Cp>经验上，把一份 300 DPI 的彩色扫描件降到 150 DPI、转成适度 JPEG，体积常能掉到原来的 1\u002F5 左右，而手机和电脑屏幕上几乎看不出差别。如果是黑白文字扫描件，还可以考虑灰度化进一步省空间。\u003C\u002Fp>\n\u003Ch2>压完还是太大怎么办？\u003C\u002Fh2>\n\u003Cp>如果单纯调分辨率仍超过收件方限制，换个思路拆或转：\u003C\u002Fp>\n\u003Cul>\n\u003Cli>\u003Cstrong>页数太多\u003C\u002Fstrong>：用 \u003Ca href=\"\u002Fdocs\u002FpdfTools\u002F\">PDF 拆分合并工具\u003C\u002Fa> 按页码范围拆成几份分别发送，每份都在限制以内。\u003C\u002Fli>\n\u003Cli>\u003Cstrong>对方只要看不要 PDF\u003C\u002Fstrong>：直接用 \u003Ca href=\"\u002Fdocs\u002FpdfToImages\u002F\">PDF 转图片\u003C\u002Fa> 把需要的几页导成 JPG 发过去，往往比整份 PDF 更小、对方也更好打开。\u003C\u002Fli>\n\u003Cli>\u003Cstrong>图片本身就糊\u003C\u002Fstrong>：扫描时分辨率就低，压缩只会更糊，这时压缩帮不上忙，只能重新扫描。\u003C\u002Fli>\n\u003C\u002Ful>\n\u003Ch2>边界：什么时候压缩没用\u003C\u002Fh2>\n\u003Cp>压缩不是万能的，这几种情况收益很小甚至为零：\u003C\u002Fp>\n\u003Cul>\n\u003Cli>\u003Cstrong>文字矢量 PDF\u003C\u002Fstrong>：从 Word\u002F网页导出的纯文字文档本来就小，里面没有大图，压完可能只小几个百分点。\u003C\u002Fli>\n\u003Cli>\u003Cstrong>已经压过一次\u003C\u002Fstrong>：对方发来的 PDF 若已被压过，再压一次画质会进一步劣化，体积却降不了多少。\u003C\u002Fli>\n\u003Cli>\u003Cstrong>含大量矢量图\u002F表格\u003C\u002Fstrong>：这类内容已是高效编码，图片重采样对它无效。\u003C\u002Fli>\n\u003C\u002Ful>\n\u003Cp>判断标准很简单：\u003Cstrong>体积大的 PDF 几乎都是图片型，压缩才有意义；体积本来就小的文字型 PDF，不必折腾。\u003C\u002Fstrong>\u003C\u002Fp>\n\u003Ch2>小结\u003C\u002Fh2>\n\u003Cp>扫描 PDF 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0h6v6\"\u002F>",{"left":4,"top":4,"width":5,"height":5,"rotate":4,"vFlip":6,"hFlip":6,"body":51},"\u003Cg fill=\"none\" stroke=\"currentColor\" stroke-linecap=\"round\" stroke-linejoin=\"round\" stroke-width=\"2\">\u003Cpath d=\"M10.3 21H5a2 2 0 0 1-2-2V5a2 2 0 0 1 2-2h14a2 2 0 0 1 2 2v10l-3.1-3.1a2 2 0 0 0-2.814.014L6 21\"\u002F>\u003Cpath d=\"m14 19l3 3v-5.5m0 5.5l3-3\"\u002F>\u003Ccircle cx=\"9\" cy=\"9\" r=\"2\"\u002F>\u003C\u002Fg>",1782539692847]