I built Mini Tool https://minitool.dev – a free, browser-based PDF toolkit where files never leave your browser with 14+ tools.

The core idea: every other free PDF tool uploads your files to their servers. For contracts, medical records,or anything sensitive that's a real problem. Mini Tool processes everything locally using pdf-lib and PDF.js running in Web Workers.

Tools included:Compress, Merge, Split, Rotate, Organize, Protect, Unlock, Watermark, Sign, Repair, Images to PDF,Booklet Optimizer,Smart Print Mode,Batch Processing, Workflow Builder.

The hardest technical challenge was getting reliable PDF processing in Web Workers across different filetypes and sizes some PDFs with embedded fonts or complex image compression would crash the worker silently.

Built this solo over several months. Would love technical feedback especially on the PDF processing approach and whether the privacy angle resonates with the crowd.