Stirling PDF is a web application you run yourself that performs the PDF operations people normally hand to a random website: merging, splitting, rotating, compressing, converting to and from images and Office formats, adding or removing passwords, filling forms, signing, watermarking, redacting, running OCR, and reordering or deleting pages. There are roughly fifty tools in the interface.
The reason it exists is the one that matters. The free online PDF converters are convenient and they require uploading your document to a third party. For a contract, a medical record, an invoice or anything covered by a confidentiality agreement, that upload is the problem, and it is a problem people commit dozens of times a week without thinking about it. Stirling PDF runs in Docker on your own machine or your own server, so the file is processed locally and never leaves.
Deployment is a single container. Many people run it on a home server or a company's internal network and point staff at an internal URL, which converts an ongoing data-leak risk into a bookmark. Optional login and user management restrict access where the instance is reachable by more people than it should be, and there is an API so the operations can be scripted into a pipeline rather than done by hand.
The OCR feature deserves specific mention because it covers a common and annoying case: a scanned document that is really a picture of text, unsearchable and uncopyable. Stirling PDF runs it through Tesseract and writes back a searchable text layer, turning an archive of scans into something you can grep.
Expectations should be calibrated. This is a utility toolkit, not a document editor: it manipulates existing PDFs rather than letting you rewrite their content like a word processor. Conversion fidelity for complex layouts is good but not perfect, as it is with every PDF converter. Running it means running a container, which is trivial for a technical user and a barrier for everyone else, and the interface is functional rather than polished.
Who it suits: anyone who handles confidential documents, small teams wanting an internal utility, and self-hosting enthusiasts. Who should look elsewhere: non-technical individuals with no way to run Docker, and anyone needing genuine PDF content editing.