base64 data to pdf

hi there , when i convert a pdf to base64 data, and then restore the data to pdf,i find that ,i must read the same  as length of base64 data every time convert ,or i can't get the right pdf. the code is:

s pdf= ##class(%FileBinaryStream).%New()
s pdf.Filename = $g(path)
s len=24*30*2
s byteList = pdf.Read(len)
while(byteList'=""){
s baseStr = ##class(%SYSTEM.Encryption).Base64Encode(byteList)

length of baseStr  is 1970,

if len=24*30 length of baseStr  is 984,

when i covert the base64 data to pdf, the code is: 

s pdf= ##class(%FileBinaryStream).%New()
s pdf.Filename = $g(path)
while ('sgc.AtEnd){
s baseStr = sgc.Read(len1)
s byteList = ##class(%SYSTEM.Encryption).Base64Decode(baseStr)
d img.Write(byteList)

when len is 24*30*2, len1 is 1970,when len is 24*30,len1 is 984, and i can get the right pdf,if the base64 data comes from others, how can i know the num of len1?