@Jerome Lai - When you say, "the guest still cannot do so", what exactly is happening? E.g., the board export menu option is not present at all? It is present, but there is no PDF option? There is a PDF option but the sub-options are greyed out? 

Next, I would ask you to confirm if your guest is accessing the board as a Guest or Visitor, i.e., you explicitly added them to the board as a Guest user using their email address or you set the board's Share setting to "Anyone with the link" and sent them a link to the board. More on these different access methods is detailed here:

https://help.miro.com/hc/en-us/articles/7045408248594