您好，微分学原理的PDF文件可以从以下链接下载：

https://www.math.ucdavis.edu/~kouba/CalcOneDIRECTORY/fundamentalsofcalculus.pdf

https://www.math.ucdavis.edu/~kouba/CalcOneDIRECTORY/derivatives.pdf

https://www.math.ucdavis.edu/~kouba/CalcOneDIRECTORY/integrals.pdf

https://www.math.ucdavis.edu/~kouba/CalcOneDIRECTORY/applications.pdf