By default, Nextcloud generates previews for text and images. Previews for other document types is
disabled for privacy reasons. Note that generating previews also require more memory and CPU.
To enable previews for PDF and OpenOffice documents, open a File Manager and edit
config/config.php and add the following setting:
'enable_previews' => true,
'enabledPreviewProviders' =>
array (
0 => 'OC\\Preview\\TXT',
1 => 'OC\\Preview\\MarkDown',
2 => 'OC\\Preview\\OpenDocument',
3 => 'OC\\Preview\\PDF',
4 => 'OC\\Preview\\MSOffice2003',
5 => 'OC\\Preview\\MSOfficeDoc',
6 => 'OC\\Preview\\PDF',
7 => 'OC\\Preview\\Image',
8 => 'OC\\Preview\\Photoshop',
9 => 'OC\\Preview\\TIFF',
10 => 'OC\\Preview\\SVG',
11 => 'OC\\Preview\\Font',
12 => 'OC\\Preview\\MP3',
13 => 'OC\\Preview\\Movie',
14 => 'OC\\Preview\\MKV',
15 => 'OC\\Preview\\MP4',
16 => 'OC\\Preview\\AVI',
),