Yes, it is completely free for both you and me.
The hosting is provided for free by Github Pages.
Short answer:
Your browser provides a tool called Webassembly, which this website takes advantage of to provide a small application that allows you to merge pdfs.
Long (technical) answer:
See README.
The process of merging pdfs is usually quite cumbersome. Either you download a (usually sketchy) application for merging pdfs, pay for a less sketchy application or use a online service to do it. Downloading an application just to merge a pdf is annoying because you usually only need once in a bluemoon, so paying for it is out of the question. And installing a sketchy application is usually out of the question, since you don't want to risk getting another virus. Using an online service to merge pdfs, requires you to upload your pdfs to their server, which is a huge invasion of privacy even if they just delete them again after merging. All these options are terrible and with the support for webassembly on all modern browsers, I thought it was time for a better alternative.
It says my pdf is corrupt, what to do?Usually it is either because it is password-protected or because of a small error in the formatting. Try opening it in a pdf-reader and save it with another name.
Where is the file?If you are on a phone the file is usually placed in your Downloads folder. You can view this by going to your Files/Archives app.
If you are on a computer the browser will usually ask you where to save the file, but if it did not, it is placed in your browsers Downloads folder.
Sometimes, especially on phones, the file is given a name that does not include the extension ".pdf". Simply renaming the file to include ".pdf" will fix this issue.
How can I contribute?If you want to help improve the project, check out the repository.