Output([string dest [, string name [, boolean isUTF8]]])
Destination where to send the document. It can be one of the following:
I: send the file inline to the browser. The PDF viewer is used if available.
D: send to the browser and force a file download with the name given by name.
F: save to a local file with the name given by name (may include a path).
S: return the document as a string.
Normally the Parameter F should only save to disk.
It looks that fpdf are forcing download.
In this case use of tcpdf https://tcpdf.org/ should be useful.
I use it and it works fine. Save to disk and insert in DB