Why is Bolding of text not working on my receipts?
If you have bolded text on your Letter - Receipt body and it is not showing the bold on the PDF being generated then it is likely that you have selected the Font Family of "Arial Unicode MS". You will need to use a different Font Family.
The Font Family can be updated on the "Letter" object.
The Font "Arial Unicode MS" with Bold in not currently supported by Salesforce. You can add your vote to Salesforce Idea exchange where this has been requested as an enhancement:Arial Unicode MS - Bold in PDF | IdeaExchange (salesforce.com)