This is because in PDF, signature fields are for adding digital signature certificates so there's not only some text or an image for signature but some special encrypted data that let's others verify the digital signature. In saying so, Adobe reader will be the best option for your intended task. To sign the PDF file without going through any tideous download or upload process, follow the steps below:
Open the target document on Adobe Acrobat Reader DC application. Now click on the fill & sign button on the right side panel.
Click on the sign button on the tool bar, then click add signature to add signature on Adobe Reader.
After executing the above steps, you'll be given 3 options to continue with creating a signature. The first option will allow you choose to type any word and have it converted to a signature. This is unadvisable as it can be duplicated by anyone one. The second is the draw option, this will allow you draw a signature using your mouse or touchscreen. The final option is selecting to a sign on a physical piece of paper, which after signing you'd have to scan using a scanner, upload and add it to Adobe Reader. This last option can be done just once as it will be stored and can be used for future works.
After creating the signature, click apply to save it on the document. Make sure to check save signature so you can easily access and add your saved signature in future.
The final step is to position the signature on the document where you want it using your mouse. After that, click save to save your signed signature. There you have it.