It is common knowledge that an employment contract should be in writing. And indeed - before the pandemic, employment contracts on paper were the most common practice among employers. 

However, in the era of the pandemic, this trend has reversed. It often happens that employers send an employment contract to a job candidate as a Word or PDF file as an attachment to an email. The candidate also accepts the content by email and starts working under it. A contract is thus effectively concluded, without either party having signed the contract in writing.

An employment contract concluded without the use of the written form will be valid and effective if only the employer signs it with a qualified electronic signature. On the part of the employee, it will be sufficient to make a declaration of will in another form, e.g. in an email (the so-called documentary form)."
 

Read more in the article by Katarzyna Sarek-Sadurska i Nicole Gerwat "Employment contract: paper is not indispensable for signing a vacancy deal " for Rzeczpospolita (article from the 9th of May 2021).