I cannot find my serial number

As Brad already mentioned, it's important to know what application or service you've paid for. If you have a perpetual license to Adobe Acrobat, you will have a serial number, and you should have received it when with your invoice or your receipt. If you have a subscription to Adobe Acrobat, then you don't need a serial number - the subscription is tied to your Adobe ID. When the installer or application asks you about the type of installation you have, you select trial/subscription and then log in with your Adobe ID.

If you only want to convert PDF documents to MS Word, it's also possible that you have a subscription to Adobe's ExportPDF service. In that case, there is also no serial number involved: This is an online service where you upload your PDF document to http://cloud.acrobat.com/exportpdf and then download the converted Word or Excel file. Again, all you need in this case is your Adobe ID.

What product did you purchase? Creative Coud apps do not have a serial number.