To be fair they buried the details of what is required deep in their Help, this is from Monster:
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What are the guidelines for uploading a Microsoft Word resume?
Last Updated:12/20/16
Please review the following guidelines before you upload a Word resume:
- Text within the body can't exceed 500 KB and the file size itself, including any images, can't exceed 5 MB.
- If you're using Word's Track Changes tool, you'll need to accept or reject all changes on your resume and save it to your computer before uploading.
- Resumes that contain viruses will not be accepted.
You can also upload a resume in PDF, RTF, or TXT format: Read how here.
You can also upload a resume via Google Drive and via DropBox.
Note: Currently, Microsoft Word 2013 documents are not supported by Monster. If you are using Word 2013, you can go to File > Save As > select the Save as type Word 97-2003 Document (.doc). This version will work on Monster.
Pages documents are in .pages format, but can be turned into usable formats such as:
Menu > File > Export > Word or PDF
In your case it will be best Exported to PDF to retain all the layout and format and because their instructions are so vague that it is unclear as to whether they require Word doc/x files older than 2013 or newer than 2013.
Fortunately for whoever created this website, they did not have to apply for the jobs themselves, because they do not seem to have a clue.
Peter