Apps similar to pdf.io include SumatraPDF, which is free and open source. Other options are Okular, Foxit PDF Reader, PDFgear and Evince. PDF Readers like pdf.io are a crowded field, with more than 50 alternatives for Windows, Mac, Android, iPhone and Linux.
Ashampoo PDF Free is a software for opening, reading, and creating PDF files. It includes a built-in printer driver for generating PDFs from any Windows application, a compact and fast PDF reader, and the ability to fill out interactive and static forms.
Germany
EU
A simple PDF document viewer, in the lines of Evince but without using the Gnome libraries. ePDFView is a free lightweight PDF document viewer using Poppler and GTK+ libraries.
Orion is a pdf, djvu, xps and cbz file viewer for Android devices, based on mupdf and DjVuLibre libraries.
EBookDroid is an open source (GPL'ed) document viewer for Android based on the VuDroid code base.
Russia
Panels is a CBR/CBZ/PDF comic reader for your iPhone and iPad devices. Enjoy reading your comics in a clean and delightful interface.
SpainEU
PDFChef enhances document management with cross-platform support on PC, Mac, iOS, and browsers. It offers text-editing, page rearranging, conversion tools, and cloud-based sharing. Supporting various file formats, it also enables users to add signatures directly to PDFs for swift document finalization.
CyprusEU
The simplest PDF viewer. Safe (doesn't require permissions), lightweight (just 6,9MB) and modern.
ItalyEU
Read, edit, annotate, and sign PDF documents with built-in AI features like summarize and translate, OCR in 50+ languages, fill forms, export to major formats offline, apply watermarks, compress files, secure with passwords, and use text-to-speech on all major platforms.
Taiwan
AI powered productivity tool for reading, understanding and taking notes on PDFs and long websites. Users can search, summarise, extract and define keywords, ask google-search type questions and be taken straight to the answers.
United Kingdom
Xpdf is a free PDF viewer and toolkit, including a text extractor, image converter, HTML converter, and more. Most of the tools are available as open source.
United States
Cloud-based PDF viewer and annotation tool for your browser. Perfect for Chromebook 1:1 schools. View, export, print and share your PDF files straight from your browser. Kami works on all browsers and works best with Google Chrome.
New Zealand