How merging without uploading is even possible
Merging PDFs is computationally simple compared to format conversion — it mostly means concatenating page objects and rewriting the document's internal page tree, which a WebAssembly-compiled PDF library can do entirely in browser memory without needing server-side horsepower.
PrivaPDF reads each file you drop in directly into the browser tab, lets you reorder them by dragging, and writes the combined output — all without a single byte of any source file being transmitted to a server.