Unfortunately pdf import isn't supported by Readlang. I recommend you use some other software to convert it to plain text first. Two approaches are:
- (Small texts) Open the pdf in Chrome or Adobe Reader, select all the relevant text and copy it. Then paste it into the Readlang Upload Page
- (Long texts) Install Calibre, use this to convert the .pdf file to .txt and then import the .txt file into Readlang.