Note that there are many PDFs that have the appearance of containing forms, but they do not actually contain any form Widgets for Xodo to find. As such, they are not highlighted in blue and tapping on the area will not bring up any dialogs to enter any data.
Above: Example of fillable forms.
Below: Example of a document that has no fillable forms.
It is possible to use the standard annotation tools to make a document without fillable forms appear filled. If your device has a stylus, simply using hand writing on top of the area where the form is will work. Otherwise, the Text Box tool can be used to create text to overlay the area where the infomation should go. This method of filling forms is more inconvenient, but the best that can be done without fillable forms. The font size and text color can be changed in the Text Box tool by tapping on the Text Box button when it is already selected. Also, the font size and color can be changed on an existing Text Box by tapping on it and then selecting style.
Text box annotations will then have to be positioned individually to align with the content of the document.