OpenDocument

Unit: Lazarus lclintf

function OpenDocument(APath: String): Boolean;

Official documentation: [1]

opendocument opens a document/file with the default viewer/editor registered with the operating system for that file/file extension. E.g. on Windows, the code will use the registry to look up the file association for the extension.

Example:

uses 
...
lclintf
...
OpenDocument('readme.pdf');