Best Free & Open Source Alternatives to Nitro PDF

What is SumatraPDF?

SumatraPDF is a free, open-source document reader for Windows built in C and C++. It supports PDF, EPUB, MOBI, FB2, CHM, XPS, DjVu, and comic-book archive formats (CBZ, CBR) in a single small executable that launches in under a second, even on older hardware. The project is intentionally minimal: it does not bundle a cloud service, an account system, OCR, or a ribbon of upsell features common to the commercial PDF readers it competes with.

Key features

• Multi-format: reads PDF, EPUB, MOBI, FB2, CHM, XPS, DjVu, CBZ, and CBR in one app, with per-format settings such as comic-book page view mode for CBZ/CBR.
• Fast and portable: ships as a single small .exe or a portable folder, no installer required, no admin rights needed, no telemetry, no ads.
• Annotations and highlights stored inside the PDF, with optional secondary sidecar .txt annotations.
• Customisable keyboard shortcuts, command-line interface, and a built-in ebook reader mode.
• Built-in PDF tools: re-flow text for tablet-style reading, favorite files and recent folders, full-screen presentation mode, and PostScript and image printing.

Comparison to paid PDF software

SumatraPDF is a lightweight alternative to Adobe Acrobat Reader, Foxit Reader, and Nitro PDF Reader. Where those products bundle cloud storage, account sign-ins, advertisements, toolbars, and large update installers, SumatraPDF is a single self-contained executable that does not phone home. The viewer supports advanced PDF features (links, forms, outlines, encryption) without any of the commercial readers' bloat. A separate companion utility, SumatraPDF Pro (commercial), adds advanced editing, OCR, and conversion, but the core viewer remains free.

What is Sioyek?

Sioyek is a free, open-source PDF reader designed for power users who work with large documents such as textbooks, academic papers, and technical manuals. It ships native installers for Windows, macOS, and Linux, and is built on top of the MuPDF rendering engine for fast page loading even on hundreds-of-megabytes PDFs.

Key features

• Smart navigation: a Quick Open dialog lets you search every PDF you have previously opened by title or path; a Table of Contents viewer jumps to any section.
• Smart Jump: click on a figure or in-text reference and Sioyek jumps to the referenced item even if the PDF does not contain internal links.
• Marks and bookmarks: vim-style single-character marks (local and global) and named bookmarks for quickly returning to a page.
• Highlights, annotations, and an overview panel that lists all figures, references, and tables in a document.
• Portals: pin two views of a document side-by-side, ideal for multi-monitor reading.
• Port-friendly installer with no telemetry, ads, or required account.

Comparison to paid PDF software

Compared to Adobe Acrobat Reader, Foxit Reader, and Nitro PDF, Sioyek launches far faster, ships a much smaller footprint, and stays out of your way with a keyboard-driven UI inspired by vi. Unlike Adobe Acrobat Reader, which nudges users toward cloud storage and paid features on every launch, Sioyek does not bundle any upsell, nor does it phone home. Its research-focused features (Smart Jump, Overview, bibliography lookup) are unique among free readers and are aimed squarely at students, academics, and engineers.

KillerPDF is a local-only, portable PDF editor for Windows built for field techs and privacy-conscious users. It offers high-quality rendering via PDFium, inline text editing with font matching, annotation tools (highlight, freehand draw, text boxes), digital signatures, merge/split capabilities, and full-text search — all without an Adobe subscription or any phone-home telemetry.

The application is distributed as a single self-installing EXE (~6 MB zipped) that targets .NET Framework 4.8. On first launch it presents an Install or Run dialog; if installed, it goes per-user to %LOCALAPPDATA% with no UAC required, registers as a PDF file handler, adds a Start Menu shortcut, and can self-uninstall cleanly. Password-protected PDFs are supported, with the decrypted copy held in temp for the session.