You application can capture PDF document files when the application is being run in a desktop browser. Mobile devices do not have a standard file system, so document file capture is not supported on mobile devices.
To capture documents, insert a column in your spreadsheet and include the word "File" in the column header. AppSheet will automatically detect this new column as a File Type after you regenerate column structure.
Alternatively, you can name the column anything you like and change the Column Type to File on the Editor>Data>Column Structure tab.