Poppler is librarified PDF rendering toolkit derived from the Xpdf 3.0 code base. It can utilize X11-independent rendering backends like Cairo, Splash, or Qt4 Arthur. It's not designed for platform-agnosticy, but proper BSD/Linux integration; is used by various PDF applications (Evince, Okular, TeXStudio, pdftotext, Zathura, Xournal, Inkscape), and comes with a set of command-line tools of its own.
wkhtmltoimage and wkhtmltopdf render HTML pages into PDF or convert them into image files respectively. They're utilizing the QT Webkit runtime for "headless" operation without a running browser or display server.
Texmaker is a free, modern and cross-platform LaTeX editor for Linux, MacOS X and Windows systems that integrates many tools needed to develop documents with LaTeX. It includes unicode support, spell checking, auto-completion, code folding and a built-in pdf viewer with synctex support and continuous view mode. Texmaker is easy to use and to configure.
OCRmyPDF adds an inisible text layer to PDF documents after passing it through the Tesseract OCR engine. The output will be PDF/A with a selectable but invisible text layer above scanned image-documents. This allows later searching and archiving.
FlaxPDF is a fast, nice multithreaded PDF viewer for the desktop. As long as there are more pages than cores, every core will get a workout. Light on dependencies, trimming borders, and aggressive caching are its major points.
TEBookConverter is an easy to use ebook converter for Windows. It utilizes Calibre. Supported input formats therefore are cbz, cbr, cbc, chm, djvu, docx, epub, fb2, html, htmlz, lit, lrf, mobi, odt, pdf, prc, pdb, pml, rb, rtf, snb, tcr, txt, txtz. Supported output formats are azw3, epub, fb2, oeb, lit, lrf, mobi, htmlz, pdb, pml, rb, pdf, rtf, snb, tcr, txt, txtz. It provides a speedy conversion process due to being multi-threaded.
pdfgrep is a commandline tool to search through PDF files. It's similar in its application to grep, and also allows searching with regular expressions or case-insensitively, and recursively through directories. It prints out filenames, page numbers, number of occurences, and most importantly a colorized match list.