I am going to try and explain this the best that I can!
When I go to open a pdf that's saved in my OneDrive, it opens up as a preview. However I can not see signatures added to the file unless I open it up even further through the browser? Is there a way I can see the signatures on the document right away without having to do the other steps?
As per the issue description mentioned above, you can't view signatures on a pdf in preview mode, is that correct?
Could you let us know if you are opening the pdf in Preview on your mac machine? If yes, please refer to the following forum thread which discusses the similar issue:
Let us know if you have a further query.