Nowadays, nearly all file formats are suitable for digital signing. There are no restrictions on digitally signed documents exchanged between information systems, but if a signed document is sent to an end-user, the following recommendations should be taken into account when choosing a file format:
- Independence from specific software: file extensions of less common software (and/or software versions with specific requirements) should definitely be avoided.
- Long-term evidential value: in the case of digitally signed containers (that include signed data files), it is important to ensure their long-term usability and evidential value. Most common file formats, i.e. those that apply as a standard, should be preferred.
- Ensuring visual unity: the data file signed must look the same to the user during signing and later viewing.
The following file formats are recommended by the guidelines of the National Archives of Estonia:
- Document formats:
- PDF in the case of formatted documents
- TXT in the case of unformatted texts
- Image formats: TIFF and PNG
- Video formats: WAVE (.wav), MPEG-1, MPEG-2, MPEG-4
The following formats are generally not recommended:
- HTML – it is not guaranteed that the user is displayed the same content as during signing when viewing the data file later
- XML – it is generally difficult for standard users to read data in XML format. XML is suitable for data exchange between information systems.
- .LNK, .PIF and .SCR files – DigiDoc4 also displays a respective warning in relation to them