我们一直把它保存在严格控制气候变化的展示台里，几乎不曾移动过；因为任何突然震动都可能使它瞬间崩溃，化为一地尘埃。
We keep it in a climate-controlled case and we hardly ever move it, because with any sudden shock it could just crumble to dust.
每当你打开一个PDF，更使一个小的变化，并保存它再次，主要看不见的事情可能会改变。
Every time you open a PDF, make even a small change, and save it once again, major unseen things may change.
它基于以下的观察：尽管页面有多种多样的变化方式，但是许多重要的页面特征在新页面都得到了保存，例如文本模式、注释信息和超级链接。
It is based on the observation that despite various changes of pages, many important page features are preserved, such as text patterns, annotations, and hyperlinks.
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