Hello Mарияс,
As per the description above, you want to change the Font style in Adobe Fill&Sign option, Is that correct?
The change in the Font Style available in Fill&Sign is not possible. Under Sign,>initials>Chnage Style> You can choose the available font styles, but there is no option to change them.
For detailed information, you can refer to Adobe article Fill and sign PDF forms using Adobe Acrobat Fill & Sign
If you have a different query/issue, please post a screenshot, to share a screenshot, please refer to https://forums.adobe.com/docs/DOC-7043#jive_content_id_How_do_I_attach_a_screenshot
Feel free to update this discussion for any further assistance.
Regards,
Anand Sri.