Being forced to log in

I don't know why this happens to some people, it's certainly bad, but why would you close your Adobe software just to look at your email? Seems much better to keep it running.

How can I stop being forced to log in every time I look at a document? 

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What kind of document -- PDF?

Which version of Acrobat do you have?  The FREE Acrobat Reader or a paid Acrobat Pro DC plan?

first, reset the cc app:

to reset the cc desktop app, make sure it's in the foreground, then press:

Win: Ctrl + Alt + R
Mac: Cmd + Opt + R

if that fails, try to repair
https://helpx.adobe.com/download-install/kb/creative-cloud-install-stuck.html

if that fails go ahead and uninstall/reinstall per the above link

if that fails, do a clean cc install following each applicable step:

uninstall every cc app including preferences, https://helpx.adobe.com/creative-cloud/help/uninstall-remove-app.html
then uninstall the cc desktop app, https://helpx.adobe.com/creative-cloud/help/uninstall-creative-cloud-desktop-app.html
clean your computer of cc files per http://www.adobe.com/support/contact/cscleanertool.html
check for, and if available, update your os
restart your computer (don't skip this)
reinstall the cc desktop app, https://creativecloud.adobe.com/apps/all/desktop?action=install&source=apps&productId=creative-cloud

if you're unable to install the cc desktop app at this stage, use the link at the bottom of the page at https://helpx.adobe.com/download-install/kb/creative-cloud-desktop-app-download.html
use the desktop app to install your cc programs/trials