Merging here means loading each file's pages into memory with pdf-lib, showing thumbnails you can drag into the exact order you want across files, then stitching everything into a single new PDF byte stream that downloads straight from your browser — no file ever touches a server in between.
There's no hard cap built into the tool, but very large or numerous files are limited by your browser's available memory since everything is held client-side.
Basic page content and images carry over intact; internal links, bookmarks, and form fields from the source files may not survive the merge depending on how they were originally embedded.
Not directly — an encrypted file needs to be unlocked first, since pdf-lib can't read pages it can't decrypt.