Unfortunately there are no Linux-based guaranteed 1-to-1 convertors for Word (doc/docx) to PDF. This is because Word, a Microsoft product, uses a proprietary format that changes slightly with every release. As it was not traditionally a publicly documented format and Microsoft does not port Word/Office to Linux (nor ever will) then you must rely upon reverse engineered third party tools for older formats (doc) and proper interpretation of the Office Open XML format by third party developers.
We found the best open source solution is LibreOffice (which was forked from OpenOffice.org, which itself was called Star Office before it was open sourced). It is much more actively developed than AbiWord, as another answer suggested.
The usage from the command line is simple and well documented with plenty of examples:
soffice --headless --convert-to pdf filename.doc
Or also you can use libreoffice instead of soffice on newer versions.
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