Thank you Edgar,
I have tried the following:
df_left_J <- left_join(PTF, ISIN_NC, by = "ID_ISIN")
I suppose I should add the mutate function...However, I see that the obs go from 233k of the starting df (PTF) to 2.009k of the final df (df_left_J). The command increases the number of rows...I just want to maintain the same inital df, with an additional column from df (ISIN_NC).