As per the issue description mentioned above, you want pdf to open at 100% zoom level by default, is that correct?
Follow below mentioned steps:
1- Launch Reader>click Edit>Preferences
2- Under Page Display>Default Layout and Zoom>Zoom>select 100% from the drop-down list>click OK
3- Relaunch Reader.
The pdf file might have initial view set in Acrobat DC which causes file to open at specific setting (page magnification, layout).
If pdf file has initial view set in Acrobat DC, default zoom will not work for that pdf.
Let us know if you have further questions on this.