How do I get rid of the big white box in front of my PDF???

Hi,

It seems like you're  using Dropbox integrated in Adobe Acrobat Reader.

Just for troubleshooting purposes, Try by accessing  your Dropbox user account using a web browser instead of the method you're using; download  the file to a known location in your computer, and try opening it with Adobe Acrobat Reader from there; not  with Acrobat Reader directly using the cloud storage service integration for Dropbox.

If this works, I can think of a few additional suggestions:

•  You may want to check that Acrobat is your default PDF viewer and PDF owner in your macOS. If it's not make it your default PDF application
• Review in Edit, Preferences, if viewing modes need to be changed, Security, Security Enhanced, and Accessibility options.
• Try some other suggestions from here: https://community.adobe.com/t5/acrobat-reader/how-to-stop-disable-adobe-acrobat-sign-in-prompt-when-opening-pdf-file/m-p/10856123#M62076 
• The white screen that is popping up could be the Reader End User License Agreement prompting the user to accept or decline. 
• https://helpx.adobe.com/x-productkb/global/unable-to-accept-end-user-license.html  
• Say if this is what is happening  see more here https://helpx.adobe.com/x-productkb/global/unable-to-accept-end-user-license.html 

Like noted above you're using Dropbox with your PDFs and this looks like you're getting an  blank Adobe Reader end user license agreement screen.

You need to relicense by accepting the EULA see here https://helpx.adobe.com/acrobat/kb/blank-eula-screen-appears-acrobat.html 

If for some reason you can't work around using this issue by opening Adobe Reader, and the Adobe guidance that I've  referenced here for you didn't work, you can manually accept the EULA  without the need of opening a misbehaving Acrobat Reader program.

This is an application preferences setting that you have to set to yes in your macOS.