Office Suites
LibreOffice
LibreOffice is popular, LGPL-licensed, free, open-source office suite available for a variety of platforms, in many languages. LibreOffice supports, and is compatible with, most Microsoft Office file formats, as well as file formats for a wide variety of other office applications from other vendors. LibreOffice includes capabilities for word processing (similar to Microsoft Word and WordPerfect), spreadsheets (similar to Microsoft Excel and Lotus 1-2-3), presentations (similar to Microsoft PowerPoint and Apple Keynote), database management (similar to Microsoft Access), vector drawing (similar to Microsoft Visio), and equation editing (similar to Microsoft Equation Editor). LibreOffice also supports exporting documents to PDF format, natively.
Free, open-source, available for many platforms
Portable Document Format (PDF)
Adobe Reader
Adobe Reader is an extremely popular PDF viewer, and supports most features available with PDF, and much more.
Free
Sumatra PDF
Sumatra PDF, also known as "Sumatra", is a free, GPL-licensed, open-source, lightweight PDF reader for Windows. Sumatra is designed to be portable, so it runs perfectly from USB thumb drives, or other storage devices.
Free, open-source
PDF Split and Merge
PDF Split and Merge, also as known as "pdfsam", is a free, GPL-licensed, open-source, tool to split, merge, and re-arrange PDF files. Pdfsam supports a variety of advanced features for manipulating PDF documents.
Free, open-source
PDFCreator
PDFCreator installs a virtual printer on any Windows system, which allows a user to very easily create a PDF from any Windows program. If you can print, you can use PDFCreator to create PDF files.
Free, open-source
Text Editor
Notepad++
Notepad++ is a free, GPL-licensed, text editor, source code editor, and overall Notepad replacement with a wide variety of sophisticated features. Such features include: Syntax Highlighting and Syntax Folding, Auto-completion, Multi-Document / Multi-View, Regular Expression Search/Replace, File Status Auto-detection, Zoom, Multi-Language, Bookmarks, Brace and Indent guideline Highlighting, and Macro recording / playback.
Free, open-source