How to pronounce "下载"
What the online community thinks

From Baidu 百科:


  准确读音 xià zài
  
  　　“下载”这个词，规范读音按照现代汉语词典、现代汉语规范词典等权威字典的标注应为“xià zài”。随着电脑的普及，这个词使用的频率越来越高，但可惜的是绝大多数人一开口就是从网上“xià zǎi”，就连播音员、主持人也读“xià zǎi”，如央视的晚间新闻在播送“网络侵权BT下载”时就读成“xià zǎi”。可以说电台、电视台的播音员、节目主持人对“xià zǎi”这个错误的读音起了推波助澜的作用。就目前的趋势来看，要推广“下载”的正确读音已非常困难，已经习非成是了。这些现象给人一种感觉，好像电台、电视台的播音员、节目主持人的发音很随意，因为这些错误的读音许多是在同一个播音员或主持人嘴里反复发出的，而这些错误的读音又严重影响了普通话规范语音的推广普及。这是当前语言文字规范化工作中不容忽视的一个问题。


It says: According to the Modern Chinese Dictionary and other authoritative sources, the accurate pronunciation should be "xià zài". But due to the proliferation of computers and rise in the usage frequency of this term, the great majority of the people when they speak, pronounce it as “xià zǎi”. This includes radio and tv hosts, and even a newscaster from CCTV. This mispronunciation has a rippling effect that from current trend seems difficult to reverse and has already become a norm. The phenomenon seems to give one the impression that the these professionals are very lax in their pronunciation. However, the mistake, in most occasions, originated from one host or one presenter, repeated over a number of times. These mispronunciations have seriously impeded the popularisation and standardization of Mandarin as the universal dialect, and is a problem that should not be ignored.

The above is further corroborated with answers to the same question here.

What a research shows

There is an interesting research paper on this “下载”的读音和释义 (http://202.194.40.31:8080/ec2006/C234/res/7/res/201010241009%5B29477%5D.pdf).

To summarize:


The two most authoritative dictionaries give different
interpretations of the same term:


  下载 xià zài: 从互联网或其他计算机上获取信息并装入到某台计算机或其他电子装置上 (跟 “上传” 相对)。
  
  《现代汉语词典》(第 5 版)
  
  下载  xià zǎi: 将互联网或其他电子计算机上的信息资料复制到本台计算机上 (跟 “上载” 相对)。
  
  《现代汉语规范词典》
  According to research [see table], most of the dictionaries indicate
  the pronunciation as "xià zài", however this is inconclusive since
  many of the minor dictionaries take reference from the more
  authoritative sources such as《现代汉语词典》and《新华词典》.
  According to a classroom survey of about 500, 68% respondents
  indicated that they pronounce the term as "xià zǎi" while 32%
  pronounce as "xià zài".
  An online poll was also conducted with a clear majority of 94%
  pronouncing the term as "xià zǎi" out of 225 respondents.
  "记载" and "装载" are the two most closely associated terms with "下载".
  The word "载" is derived from "车" and "装载" precedes "记载" which is
  derived from "containing" in a metaphorical sense.
  
  The above are some points put forth by the author, I left out part of his argument and conclusion which I find subjective and highly debatable.


Justifications

The use of "xià zǎi" can be justified to be derived from the term "记载" (jì zǎi) since "下载" or download refers to receiving a copy of the original file on the local system. The action of copying can be taken to mean recording or "记载" although it is not so obvious to some.

The use of "xià zài" can be justified by the single word "载" (zài) which means transporting. Downloading is a process of transmitting (or transporting) electronic data from the remote system to the local system.

The verdict

Inconclusive. Use whichever pronunciation that you feel can get the other party understand you.