This PPTX to PPT conversion does not run here
A few old systems still ask for .ppt rather than .pptx.
Browsers cannot write the legacy .ppt binary format.
What to do instead
Open the PPTX in PowerPoint or LibreOffice and use Save As. Or send a PDF from PPT to PDF if the other person only needs to view it.
The page that does work is PPT to PDF.
When PPTX to PPT is the right page
PPTX to PPT sits on this site because PPTX and PPT are not interchangeable, even when they look similar in a file list. A few old systems still ask for .ppt rather than .pptx. Decks are shared as PPTX when the other person will present, and as PDF when they only need to read. People search for PPT when an old machine or a course portal names that extension.
Browsers cannot write the legacy .ppt binary format.
What you give up by leaving PPTX
PPTX is a modern PowerPoint file. The design lives in the file. A text export only keeps what was written on the slides. Themes, animations, masters, and embedded video are not cloned by a browser tool.
Keep the PPTX. A PDF or Word file of the words is not the deck.
What you gain, and do not gain, with PPT
The download has to be PPT, because that is the format the next step named. People search for PPT when an old machine or a course portal names that extension. A PPTX can sometimes be re-saved if it is already the modern file with the wrong expectation.
Do not expect animations or old templates to be rebuilt in the browser.
A test that matches the real file
Start from the question the other software asked. If it asked for PPT, PPTX to PPT is the matching page. If it asked for something else, switch tools instead of hoping a similar extension will pass. A few old systems still ask for .ppt rather than .pptx. Browsers cannot write the legacy .ppt binary format. The limit is the part people skip, and it is the part that decides whether the download is usable. Open the PPTX in PowerPoint or LibreOffice and use Save As. Or send a PDF from PPT to PDF if the other person only needs to view it. The file stays in your browser and is not uploaded. The limit is 100 MB. Keep the original until the download has been opened in the program that will use it. Check three things in the download: it opens, the PPTX material you cared about is still there, and the file size or the layout is what the form described. A new extension alone is not those three things. If a portal, a teacher, or a client rejects the file, send them the original PPTX as well when you are allowed to. PPTX to PPT is a convenience copy. It is not a new master. Slides are a design. PPTX to PPT can take the words off them or move a file a browser understands. It does not copy animations, masters, or embedded video. When the deck itself is what you will present, export from PowerPoint or LibreOffice. Use this page for a handout or a format a browser can actually write.
Here is the decision for PPTX to PPT. You start with PPTX and you stop when a real program accepts PPT. Slide text can be pulled out for a handout or a transcript. A real PowerPoint 97 binary is rejected here. Open it in PowerPoint and Save As PPTX. If those two sentences describe a tradeoff you do not want, do not convert. Renaming the extension by hand will not do this job. Running the tool twice on its own output will not do it either.
Open the PPTX in PowerPoint or LibreOffice and use Save As. Or send a PDF from PPT to PDF if the other person only needs to view it.
A related step, if you still need one, is PPT to PPTX.
Other jobs start from the homepage, where each tool is named for the conversion it actually does.