Office

PDF Reader

Pdf Viewer Plus

Pdf Viewer Plus is a smooth PDF viewer written in Java. It contains a simple user interface, multiple themes, and the ability to share a PDF.

Librera PRO

Librera Reader is a highly customizable and feature-rich application for reading e-books in PDF, EPUB, MOBI, DjVu, FB2, TXT, RTF, AZW, AZW3, HTML, CBZ, CBR, DOC, DOCX, etc. formats

MiXplorer (Closed source)

Integrates an excellent reader for PDF documents

Office Apps

There are no really good alternatives yet. You can try these apps, but they don't seem to be totally reliable: Collabora - AndrOpen - OpenDocument - LibreOffice Viewer

My advice is to convert the various documents to PDF, if you have to edit them, use classic closed source apps.