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LibreOffice
Read-only LibreOffice core repo - no pull request (use gerrit instead https://gerrit.libreoffice.org/) - don't download zip, use https://dev-www.libreoffice.org/bundles/ instead (by LibreOffice)
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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
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pdfarranger
Small python-gtk application, which helps the user to merge or split PDF documents and rotate, crop and rearrange their pages using an interactive and intuitive graphical interface.
The free and open-source version of PDFsam supports merging files into a single PDF (and several other actions). It looks like PDFsam Enhanced supports insertings watermarks, comments, etc. depending on the plan.
I believe LibreOffice may be a suitable solution to your situation especially for multiple users/office.
For merging, check out PDF Arranger. If you need to turn those emails or anything else into pdf, you can use any pdf print driver, and then put it all together with pdf arranger. For the stamps, LibreOffice Draw should do the trick. It won't be an automatic one click type of thing like i'm guessing it is in Adobe, but it's free.