The page downloads a small PDF library. When you pick a file, your browser reads it and builds the new PDF locally. The document is not sent to a server.
Very large files can be slow on older phones. Heavy jobs such as password protection, OCR or exact-layout PDF to Word need more processing than a browser offers for free. BrightPrintableHub PDF Tools’s Word, Excel and Word to PDF tools are basic text-only versions, and the rest are listed as coming soon.