What actually distinguishes free PDF mergers
Most free merge tools do the basic job fine; the differences show up at the edges — whether there's a file count limit, whether the output gets watermarked, and whether your files are uploaded to do something as simple as concatenating pages together.
PrivaPDF has no file count cap (beyond your device's memory), no watermark on the free tier, and no upload step at all — the files stay local from drop to download, which for a tool this simple is arguably the more meaningful differentiator than any feature comparison.