Researchers and academics accumulate thousands of PDFs with useless filenames like 1706.03762.pdf or download(3).pdf. The HN Ask HN thread makes the pain explicit: someone directly asks for a tool that finds metadata of PDF files based on hash or content, 'Like MusicBrainz Picard for identifying and tagging mp3 files.' That analogy is the entire product thesis. PDF Metadata Tagger is a desktop and cloud tool that fingerprints PDF books and papers by content, matches them against open metadata sources, then auto-tags and renames files into a consistent, human-readable scheme. Solution: a background agent scans a folder, extracts text and structural fingerprints, resolves each file to a canonical record (title, authors, year, DOI, publisher), writes clean metadata into the PDF itself, and renames files per a user template. MVP: a local-first CLI plus lightweight GUI that handles a watch folder, uses free open sources (Crossref, OpenLibrary style DOI and ISBN lookups) for matching, and supports a rename template. Revenue: freemium SaaS. Free tier caps monthly identified files; a paid tier (individual researcher) unlocks unlimited batch processing, cloud sync of your metadata library, and integrations with Zotero and reference managers. GTM: seed the exact HN thread and subreddits where this demand already surfaced, plus academic Twitter and lab Slack channels. Growth loops: a shared open metadata correction layer (users confirm or fix matches, improving the match graph for everyone, MusicBrainz style), plus referral via shared reading libraries within research groups.