Margins setting print poster

Hi everyone,

I am trying to print a document in as a poster so I can have a document in an A2 format using two A2 papers. Its my first time doing this, when I choose the "Poster" mode it change the size of my file initially set as A2 42 cm*59,9 cm with PowerPoint that I download as a PDF. When I click on printing, than poster in Adobe Acrobat it change it directly 83,9cm * 59,3cm, why is this 100% ?

And in addition it add large white "margins" all around the document that are impossible for me to change, I tried to change it with the mosaic scale but than it also change the size of the document. It does look like a poor adjustment like the document where not created in th

Sorry if I am not very clear. 

Thank you for reading and maybe being able to help.