Free and Open Source Software

VSCodium (or any other flavour) with the PDF Viewer plugin will allow you to browse and view PDFs.

Is this the sort of thing you're looking for?

https://www.files.gallery/

It's a php file you drop into a folder that creates a gallery of the files (including pdfs). There are security settings and visual customisations you can change. If you know what you're doing you can access the file remotely, too. I've used it for a while and found it helpful.

Maybe I would

1. Create a file called pdfs.html
2. Add embedded PDFs

   <embed src="pdf1.pdf"></embed>
   <embed src="pdf2.pdf"></embed>
   <embed src="pdf3.pdf"></embed>
   

3. Maybe give it a container div with display grid for min size and useful layout
4. Open the HTML file in my webbrowser Firefox

To generate the embed codes for every PDF file in a folder I would use my command line shell Nushell, which generates the embeds for all files for me.

ls *.pdf | each {|x| $'<embed src="($x.name)"></embed>'} | str join

KDE's Dolphin (a file explorer) can show previews of PDF files.

What do you mean by “open multiple files at once” and “viewing a directory”?

Emacs can easily do that pdf viewer and one of the several tree folder packages.

https://github.com/makerj/vim-pdf