For the safe use of an Identity Card (ID-card), residence card and Digi-ID:

• Please do not hand over your ID-card, residence card or Digi-ID to anyone. It is possible to misuse the card, but still you will be held liable for it.
• Please keep the codes of your card secret from others. Do not write any codes on the card and do not keep the codes together with the card. Store the codes in secure place or, even better: learn them by heart and destroy the envelope of the PIN-code(s).
• Do not keep the ID-card in the card reader just so. After you have finished using the ID card, always remove the ID-card from the card reader. 
• After you have finished the operations, please log off the online service and close the web browser. Web browsers can temporarily remember PIN1-code, therefore the next user of the computer may succeed in logging into some web service under your name without entering the PIN1-code.
• If a card has been stolen or is lost, please make a prompt call to the help-line phone number 1777 (from abroad +372 677 3377) for suspension of the certificates, and, for invalidation of the document, please address the closest Service Office.
• If the PIN or PUK codes of a card have been stolen or are lost, please address the closest Service Offices or a bank office which provides additional services for obtaining new codes. More information is provided here.
• Please remember, that a digital signature given by an ID card, a residence card or Digi-ID is equal in strength with the manual signature and that it will involve liability. 
• PIN2 code is equal in strength with your signature. Before using the PIN2, please make sure, that the other party is trustworthy. In case of doubt, please verify, for example, the correctness of his or her document. 
• If possible, please change your PIN1, PIN2 and PUK codes promptly after receipt of the card. 
• Do not change the PIN1 and PIN2-codes into similar ones for the sake of being easily remembered.
• Keep your computer clean and your software updated. Always use security software in your computer. Download the updates which are offered by your operating system, security software, web browser and ID card manager. Think over, where or whom you should approach if your computer or phone is acting suspiciously. 

Useful links (in Estonian)
Renewal of the certificates of an ID card
Where can I use the card
Terms and conditions for use of the certificates

Statutory acts (in Estonian)
Digital Signatures Act
Identity Documents Act

Digital signature counterparties in Estonia
State register of certificates
Certification centre 

Digital signature and ID card abroad
Finland
Sweden
Europe