I am creating mailer postcards. Which program is best?

I don't know if I even have the correct 'community' to ask this question.  I am new to marketing, a late bloomer, but enjoy the design aspect and learning.  I am working with a friend who sells ads for a mailer postcard.  He has asked if I could help put it together for his new printer.  Some of the files come to me as a pdf, some as a jpeg, some as word.  I have been using Publisher for my main template, and then Acrobat to convert the files to a jpeg to insert and turning back to a pdf for the printer.  I subscribed to Acrobat thinking maybe I should be creating it there instead of Publisher.  I am going through online classes to learn Acrobat, and am beginning to think maybe that isn't the appropriate app to use.  I have already done 3 postcards, and his printer says they weren't done correctly, first saying I needed to flatten it, then convert RGB to CMYK (I figured out both of those). Someone else mentioned raster vs vector, which I know nothing about.  Any and all suggestions are greatly appreciated.  Thank you.