It took me ages to figure out and many articles on here pointed me in the right direction but didn't quite work for me.
I use Word 2016 for Mac and Abode Acrobat Reader DC.
To create a PDF that retains the custom page size:
- Highlight all of the text in the MS Word document.
- File > Page Setup> Page Attributes drop down box, choose Microsoft Word.
- For the 'Apply page setup setting to:' field, select 'Selected Text'
- Then immediately go to File > Print
- Click on 'Copies and Pages' drop down box and select 'Paper Handling'
- Select 'scale to fit paper size'
- Choose ‘Custom’
- Then click on PDF (bottom right hand corner)
- Click Acrobat Reader (if installed).
- To check that it has worked, open the document in Adobe Acrobat Reader.
- File > Properties
- It should say at the bottom in the Advanced section what the page size is.
- To change the page size from inches to mm, click on Acrobat Reader at the top next to File.
- Go to Preferences > Units.