这个是我在国外的一个网站找打的，原文地址，Maximum File Size for Crawling具体的修改办法如下：
Maximum File Size for Crawling
By default, Search Services can crawl and filter a file with a size of up to 16 megabytes (MB). It will always crawl the first 16MB of a file. After this limit is reached, SharePoint Portal Server enters a warning in the gatherer log “The file reached the maximum download limit. Check that the full text of the document can be meaningfully crawled.”
To increase the limit of 16 MB, you must add in the registry new entry MaxDownloadSize. To do this, follow these steps:
1. Start Registry Editor (Regedit.exe).
2. Locate the following key in the registry:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office Server\12.0\Search\Global\Gathering Manager
3. Open Edit - New - DWORD Value. Name it MaxDownloadSize.
4. Double-click, change the value to Decimal, and type the maximum size (in MB) for files that the gatherer downloads.
5. Restart the server.
6. Start Full Crawl.
NOTE: Increasing the file size may cause a timeout exception because the crawler can timeout if the file takes too long to crawl/index (because of its size). To increase timeout value, follow these steps:
1. In Central Administration, on the Application Management tab, in the Search section, click Manage search service.
2. On the Manage Search Service page, in the Farm-Level Search Settings section, click Farm-level search settings.
3. In the Timeout Settings section change Connection and Request acknowledgement time.