Which plugin is used for viewing PDF documents?

The choice of Adobe Reader as the correct answer is based on its specific purpose and functionality related to viewing PDF documents. Adobe Reader is a dedicated application developed by Adobe Systems for the purpose of opening, viewing, and printing Portable Document Format (PDF) files. It supports various features, such as annotations and form filling, which are relevant for interacting with PDF content.

In contrast, ActiveX is a technology that can be used to embed interactive content within webpages, but it is not specifically designed for viewing PDF files. Microsoft Word is a word processing application that can open PDF documents but is primarily intended for creating and editing text documents, not for reliably viewing PDFs in their original format. QuickTime is a multimedia framework used for handling video and audio content, and does not serve the function of viewing PDF files. Therefore, Adobe Reader clearly stands out as the dedicated tool for PDF document viewing.