Not just viewing — editing too.

A DOCX editor (styles, bold/italic, alignment; opens .odt too), Excel/CSV spreadsheets with formulas (SUM, AVERAGE, IF, CONCAT…), a PowerPoint viewer with presentation mode, a slide editor, a text and code editor (find-and-replace, HTML preview), Markdown and checklists, and a ZIP viewer. Write on PDFs with a pen, highlighter and text; delete, rotate and reorder pages.