I think the problem is very old. i found it in many posts in adobe fourm. but the strange this that it isn't solved at all. where in other simple apps it has just small option to solve it.
for more info. please check this

https://community.adobe.com/t5/acrobat-reader-mobile/cannot-write-files-pdf-on-microsd-card-samsung-tablet/td-p/8876939?page=1

https://community.adobe.com/t5/acrobat-reader-mobile/android-copy-in-reading-mode/m-p/9163372?page=1

https://community.adobe.com/t5/acrobat-reader-mobile/acrobat-android-7-0-save-overwrite-pdf-on-sd-card/td-p/8994684?page=1

https://community.adobe.com/t5/forums/searchpage/tab/message?advanced=false&allow_punctuation=false&filter=location&location=forum-board:acrobat-reader-mobile&q=read%20only%20file%20on%20android

How to Solve it. Please just add small options like this app

http://feedback.xodo.com/knowledgebase/articles/752619-accessing-files-on-an-sd-card