![]() Some text can be hidden behind the images which again can be extracted. For example, white text on a white background (or other text of any color that is matching with the background color) could be hidden, but still, be extractable/ copyable if all the contents are copied and pasted into a notepad or extracted through a PDF text extractor. ![]() **Concealed Text and Image:** Text can be concealed in numerous ways. PDF optimization will remove inefficiencies and hidden data risks pertaining to single pass or incrementally updated files. Hence, the recommended sanitization approach includes regenerating the file using the PDF optimization procedure given by Adobe Acrobat. **Data Retained Even After Updation:** Updating PDF files incrementally, the previous versions of data may still be retained, but it is not visible to the user. Yet, when the document is created and ready to be distributed, it may still retain data or information not intended to be shared with the target audience. **Reviewing and Commenting:** Commenting and reviewing features are commonly used when you are collaboratively preparing documents. It will help in maintaining the appearance of the file while reducing the risks associated with forms. Hence, the recommended sanitization approach outlined in this article recommends flattening all form fields (as well as eliminating any mechanism or scripts associated with the forms). Also, communication mechanisms that are used to transmit this data could create hidden data risks. Form fields store user data, but form processing information may contain information that is not intended to release beyond certain organizational boundaries. **Interactive Form Data:** It is basically a PDF form. That is why it is important that embedded search indexes are removed before distribution. But sometimes these search indexes may retain content from previous versions of the document that has been removed either it was not required or it was ill-natured. **Embedded Search Index:** It helps in faster and easier search of information within a PDF file for users, especially if they are working with a large PDF file. Another thing you can do is remove scripts and embeddings. In order to sanitize the hidden layers, we recommend flattening layers so that all data or graphic images are displayed in a single layer. Some non-visible layers may contain malicious embeddings that the other users might not want to retain. ![]() Let me say this, layers are highly difficult to effectively evaluate. A button can be used to control which component is displayed in that same graphic image area. In many cases, this might also be tied to JavaScript to control layer visibility. ![]() This technique is mostly used in architectural and engineering designs to view different angles or components of a complex object in the same context. **Hidden Layers:** Using layers enables authors to include multitudinous representations of content within a single area. Types of information that may be accidentally or intentionally released include system data, network attributes, and business process data. **Scripts:** Applying actions through scripts/ Javascripts may enhance user experience but they also may contain “more information” than the author of the PDF intends to publish. It may be directly attached to the PDF file or may be present in a PDF portfolio. **Attached Files:** A PDF file has some external file or also known as attachments that can be of any file format. **Note: You can extract metadata of any PDF file using** ( ) It’s an important element for PDF forensics. Again, XML has a standard platform called Extensible Metadata Platform (XMP) that helps in embedding metadata in digital files such as PDFs, photos, videos, etc. It is represented in Extensible Markup language (XML). It helps in organizing a whole PDF library and retrieving information. Talking about the fields, it contains Document Title, Author, Subject, Keywords, and Copyright information. **Metadata:** It contains searchable fields and can be accessed by any search utility.
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