Conventional data hiding methods for PDF file insert a payload data by slightly modifying the position of characters in a document. Even if the changes are small, a certain degree of visual distortion is inevitably introduced to the PDF file. In this work, we propose a new data hiding method that splits the space value between characters. Specifically, a space value is split into two or more related values. Except for the first value which is reserved to store the corrective data, each of the related values encodes a segment of the payload data. When the PDF file is opened by a PDF viewer, the visual appearance is exactly the same as its original counterpart, i.e., complete quality preservation. To prevent direct observation of PDF file, access control is introduced by setting an owner password, which is a built-in function in the PDF standard. In the best case scenario, 38,160 bits can be hidden, while the observed file size increase is 12,776 Bytes.

PDF文件的传统数据隐藏方法通过稍微修改文档中字符的位置来插入有效载荷数据。即使变化很小，也难免会在 PDF 文件中引入一定程度的视觉失真。在这项工作中，我们提出了一种新的数据隐藏方法，可以分割字符之间的空间值。具体来说，一个空间值被拆分为两个或更多相关的值。除了保留用于存储校正数据的第一个值之外，每个相关值都对有效载荷数据的一段进行编码。当 PDF 文件被 PDF 查看器打开时，视觉外观与其原始副本完全相同，即完全保持质量。为了防止直接查看PDF文件，通过设置所有者密码来引入访问控制，这是PDF标准中的内置功能。