PDF (Portable Document Format) is a file format that fixes the exact position of every character, line and image on a page, so a document looks identical whether you open it on a phone, a laptop or a print shop's RIP. Adobe created it in 1993 and handed the specification to ISO in 2008, where it became the open standard ISO 32000. That openness is why so many independent tools can read and write PDF without asking anyone for permission.
Under the hood a PDF is a tree of objects: page descriptions, fonts, images and a cross-reference table that lets a reader jump straight to any object without parsing the whole file. Because the structure is well documented, a PDF can be edited entirely in code, and that is exactly what reader.me does. Every operation runs as JavaScript and WebAssembly inside your browser tab.
That local-only approach matters for privacy. A contract, a medical report or a payslip never leaves your device, so there is no upload, no server-side copy and nothing to delete later. The file you started with and the file you download are the only two copies that ever exist.