There are two mail clients that do this.
Outlook
Microsoft then may have built in their own custom attachment viewers within Outlook for their own Word, Excel, & PowerPoint programs they wrote themselves.
eM Client doesn’t write those type of Outlook viewers built-in. Microsoft might have them possible down the track as an eg: API for other developers to integrate to other mail clients outside Microsoft. Who knows.
You would need to ask Microsoft directly about that.
Acrobat PDF files opens perfectly in eM Client already with Adobe acrobat reader installed in eM Client or the pdf reader built into Edge in Win 10 / 11.
I open pdf and jpg & Office files using eM Client with the default stuff built in Windows & Libra Office for free unless you already own office, but you can obviously install other advanced readers and jpg viewers etc optional apart from the OS.
Thread on the topic
.https://forum.emclient.com/t/preview-and-print-attachements/71315
As @Gary posted in the thread above.
Only some attachments like images and eml files are previewed in-line. That means the attachment will be displayed below the message text.
Some others like PDF and HTML files only give a small thumbnail preview when you hover the mouse over the attachment.
There is no indication as to whether all attachments will be previewable inline in the future
