How to remove “YEAR/MONTH/” from uploads URL & redirect old URL to the new form?

Question

The .htaccess RewriteRule to rewrite only .pdf to not use /year/month/ can look like this

RewriteRule ^wp-content/uploads/([0-9]+)/([0-9]+)/(.+?)$ /wp-content/uploads/$3.pdf [L,R=301]

You should add this prior to WordPress’ rules, so it will be evaluated before them, resulting in a .htaccess file like this

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^wp-content/uploads/([0-9]+)/([0-9]+)/(.+?)\.pdf$ /wp-content/uploads/$3.pdf [L,R=301]
</IfModule>

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress