在 PDF 中添加一行和修改已有的行是两种不同的操作，尽管在屏幕上看起来是一样的。前者是绘制新内容，而后者则需要深入到已绘制的内容中——即一组具有特定字体、位置和间距的指令，而其中的字体通常是以部分副本的形式嵌入的，仅包含该页面所使用的字符。所以我的原则很简单：只修改变动部分，其余保持原样。编辑的目标是尽可能小的变动范围，并在保存之前将结果与原件进行比对——如果页面出现了不应有的偏移，则完全不执行此次编辑。详细版本请参阅 Guides：“如何在 PDF 中编辑现有文本”。✎
Adding a line to a PDF and changing a line that is already there are two different operations, even though on screen they look like the same thing. One draws something new. The other has to reach into what is already drawn — a set of instructions with their own font, position and spacing, where the font is often embedded as a partial copy carrying only the characters that page used. So my rule is short: change what you changed, leave the rest alone. The edit aims at the smallest thing that has to move, and the result is checked against the original before anything is kept — if the page moved in a way it should not have, the edit does not happen at all. Longer version in Guides: "How to edit existing text in a PDF". ✎
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