Firstly, I should point out that 300dpi is total overkill for internet publishing. 300dpi is the quality often used for commercial printing (e.g. printing presses, etc). Depending on the number of pages your PDF documents are, filling them with 300dpi images will also add considerably to their file size.
Secondly, if you're finding that the quality of PDFs produced by MS Word is such that the images do not meet your standards, then the problem may be with your original images. Are they of the quality you need? If the original images don't meet the standard, then their quality is not going to upscale very well.
All that said, if you only have about twenty documents to produce, you could opt to just take advantage of a free Adobe Creative Cloud trial. You could then use Adobe Acrobat Pro DC that will allow you to either print to Adobe PDF, or to export using the Adobe Acrobat plugin that is installed in MS Office. Either way you will be able to produce PDFs of varying quality (even press quality PDFs used for commercial printing). This option also preserves any hyperlinks you have.
Otherwise, some popular open source options are:
NOTE: I can't personally vouch for the above open source options as I'm an Adobe Acrobat user, but I do know of people who are happy with them.
[EDIT]
I forgot to mention LibreOffice. You could try its LibreOffice Writer app to either:
- open one of your MS Word docs and export to PDF to do a comparison of the quality
- create a test document including a couple of hyperlinks and a couple of images and export to PDF to compare quality