Text Callout arrow lines show as triangles

Hello everyone.

I am having this problem with Text Callouts. All of them are created and printed on PDF as they are supposed to be.

Example

However,  when theese PDFs are viewed in another system, all the arrow lines are shown as triangles.

Example:

Is there any setting that the viewer should chanage in order to see the arrowas as arrows and not astriangles?

Any help wwould be much appreciated

Thank you .