PoDoFo is a library to work with the PDF file format. The name comes from the first letter of PDF (Portable Document Format). A few tools to work with PDF files are already included in the PoDoFo package.
The PoDoFo library is a free, portable C++ library which includes classes to parse PDF files and modify their contents into memory. The changes can be ...
Two free software candidates would be:
Imlib2 - image loading and rendering (however, no longer maintained)
ImageMagick, versatile image processor, with command-line interface and various APIs - the C API is MagickWand
The frameworks you mentioned will bring you a lot of tools and support, which is important if you want to create professional web applications. jQuery is a bit different as you can pick what you need and leave the rest unused. This only will bring you some overhead.
If your target is small applications for internal use, maybe the "big" frameworks ...
RegExr describes itself as an online tool to learn, build, & test Regular Expressions:
Results update in real-time as you type.
Roll over a match or expression for details.
Validate patterns with suites of Tests.
Save & share expressions with others.
Use Tools to explore your results.
Maybe have a look at: https://freefem.org/
I used it to solve Poisson on 2D and 3D unstructured meshes using finite elements.
They had implemented some MPI some time back:
The team that developed this solver is still active (researchers from JLL lab of University Paris VI).