The problem is usually caused by the receipt of incorrectly formatted mail. This is especially a problem with mail from "non meil clients" as it is often created by developers with little or no understanding of the processes or the MIME standard used to encode attachments. Thunderbird will display attachments that are misconfigured under some circumstances. But it will not forward them.

Try using the forward as attachment option. That will attach the mail you are forwarding to a new email and should retain the attachment.

The problem is clearly with the workplace. Because I decided to check and sent a pdf file from MS Outlook 2013 to two recipients and tried to forward my message with these files to these recipients. In the problem place (screenshot 1), the pdf file is not attached, and on the other (screenshot 2) it is attached. Also, the problem remains when sending mail from the "Sent" folder - the pdf file is not attached (screenshot 3). With other types of files there is no such problem. I think the problem is with Thunderbird and somehow it can be solved, only I do not know how. The option to forward the message itself as an attachment is not suitable.

These are some of the bugs, your not going to fix it here. The issue is with the sent mail being malformed.

https://bugzilla.mozilla.org/show_bug.cgi?id=408114 https://bugzilla.mozilla.org/show_bug.cgi?id=548507

Or the more general issues with MIME encoding and the strict way Thunderbird applies it. https://bugzilla.mozilla.org/show_bug.cgi?id=1405234

If you create a new Windows account and copy the folder "% userprofile% \Appdata\Roaming\Thunderbird" from the Windows account where there is a problem, then everything works in the new Widnwos account, i.e. dpf attachments do not disappear upon forwarding. I conclude that some settings in the Windows account, where there is a problem, interfere with the normal sending of letters with pdf attachments. Creating a new Windows account and copying the Thunderbird profile there solves the problem, but this is not always acceptable, because You have to reconfigure the Windows account for the user. I would like to figure out what kind of setup or problem prevents Thunderbird from working properly.

I do not know any more than I have posted, but does you new windows profile have the same anti virus settings? They are quite good at making issues with doc and pdf files. Mostly because the treat them as executable files and scan them as such.

The settings are the same, because it is the same PC. I tried it with a disabled antivirus - no changes.