That 100MB ceiling (100 * 1024 * 1024 bytes) comes from a maxFileSize option passed straight into formidable's own upload parser, not from anything specific to .doc or .docx.
The server has no .doc or .docx allow-list, but checks such an upload's first bytes against its format signature before Word To PDF hands it to LibreOffice.
Word To PDFWord To PDF Doc Docx File Size Limit
Microsoft's support forum documents a Word file failing to export to PDF once it grows past roughly 20 to 40 megabytes. 365 Word file too large for PDF - Microsoft Q&A
Select the .doc or .docx file from your device.
Review the file size before submitting the form.
Checking Conversion Results for Word Files
Download the finished PDF from the results screen.
Open the downloaded PDF to inspect the layout.
Continue with Word to PDF Fonts Formatting Changed or Word to PDF Batch Convert Multiple Documents.
Frequently Asked Questions about Word to PDF Doc Docx File Size Limit
How do I upload a legacy .doc file?
Select the .doc file from your device and submit the form.
Where do I find the converted PDF?
Download the finished PDF from the results screen.
How do I check the file size before uploading?
Review the file size in your file manager before selecting it.
What should I do if the conversion fails?
Check the file size and try uploading a smaller version of the document.