The first release of the PlusCal rendering engine was published in September 2004. Since then, PlusCal has been regularly updated with new features and bugfixes. Still, the PlusCal rendering engine is a free software program.
The free software suite Xournal is a simple, easy-to-use note-taking application that's perfect for sketching, sketching, and more. Xournal supports a wide range of file formats, and it's also lightning fast.
libreoffice, the Free LibreOffice fork with many changes; no patent encumbrances. LibreOffice 5.4.2 was released in March 2020. The website no longer redirects to the LibreOffice web site; the LibreOffice download is now located on the website.
Acrobat is a popular PDF creation and editing tool used in Adobe's Creative Suite. A free, open-source alternative is called Okular.
Okular supports annotations and highlighting, a feature not found in Acrobat.
Okular can also be installed alongside Adobe Acrobat and offers a similar interface.
There are also other Linux-based alternatives to Adobe Acrobat, including:
PDF Edit - available for Windows, Linux, and Mac OS. It can be installed on Windows without Adobe Acrobat, and can view, modify, and print PDF files without restrictions.
PDFium - a fork of PDF Edit. It is available for Windows, Linux, and Mac OS.
Tika - a Java library for creating and manipulating PDF documents. It supports PDF annotation, PDF print, OCR, form fields, form navigation, and file import.
Apache pdfbox - a Java library for working with PDF documents. It can read, write, edit, and manipulate PDF documents.
Apache poppler - another Java library for working with PDF documents. It can read, write, edit, and manipulate PDF documents.