Yesterday I met firstly the word 'pal' in a crossword puzzle - as a synonym for 'friend'. Since I didn't know this word, I'd like to know if I can use it or I'll sound weird. How real is the word 'pal' as a synonym of a 'friend' common or acceptable?
It is a real word. In written English it is much less common than "friend" but about as common as other informal words for "friend" such as "buddy" or "mate" (ngrams) (the ngrams data for "mate" is likely skewed by the verb "to mate", but Buddy is also used as a name and pal might be an abbreiviation P.A.L.) All these words would be more common in spoken English. (and "mate" is particularly a British/Australian word)
Pal has a particular use in the compound "pen pal" a person (often in another country) with whom you communicate by letter only.
Jake was learning Spanish, so he found a pen pal in Mexico, and emailed him about once a month.
It may interest you that this is a word of Romani (gypsy) origin "ph(r)al = Brother" (although this meaning has been lost in English)
You may run the risk of sounding like a geriatric using it. There is an element of nostalgia to using it effectively, e.g reminiscing about your school days with your old pals.
You could quite easily substitute chum for pal as well, it's very British. Interestingly in Australia, "Pal" and "Chum" are also the names of popular dog foods, this is perhaps no coincidence as dog is man's best friend.